First, there is the very clear "literal" aspect of the line. Hannibal is directly commenting upon the reliability of Chiyoh, and her consistency. We know that for an exceptionally long time, Chiyoh was the guardian to Mischa's alleged killer. She was bound to this man not out of necessity, but out of her own morality which forbade her killing him. While ultimately she did kill Mischa's killer, this was largely as a result of Will Graham's interference, and was not her intended outcome. We also know that she had a degree of friendship with Hannibal. Throughout the season, we got a variety of suggestions as to what her motivations were; was she here to kill Hannibal after Will Graham suggested Hannibal killed Mischa as all sorrows can be borne through a fairytale (not the exact quote, I know), was she indebted to Will for 'freeing' her, was she going to kill Hannibal, would she assist in his arrest, would she help Hannibal, why was she doing what she was doing, etc. It is in this episode that we discover it very simply; she is (as Hannibal mentions) obsessively and successfully driven by a debt to Mischa and little else. She watches over Hannibal and will serve as his protector so that she can get an answer to the doubts that Will Graham implanted within her, and so that she is able to 'honour' Mischa. DuMaurier comments in the previous episode that she worries that Chiyoh is Hannibal's greatest mistake, and the reason is not because of how damaged she is, or how clever she is, but in how obsessively she will patiently hunt down what it is she seeks with a reckless disregard for her own personal safety (note how she carries around a sniper, how she waltzes with a drive, how she takes action even in public places or against connected individuals, yet doesn't intervene when Jack Crawford appears in Florence despite it being a good moment to catch Hannibal, she doesn't cause collateral damage unnecessarily, and doesn't kill when unnecessary). The name "Chiyoh" means eternal, and her focused, driven nature is something "stable" that Hannibal can always rely on.
Then there is the clearly metaphorical meaning. It doesn't, as you've suggested, imply behind bars, as at this stage, Hannibal has not turned himself in, nor does he have any intention of doing so. "Iron" and "Silver" are the key words in this statement, so it's important to consider what these could represent. Let's look at the word "iron". It has connotations of strength, endurity, hardness, coldness, and resilient. It won't bend, and it won't break. It's reliable. Yet it's also relatively cheap, and associated with many cheap materials ("iron pan" versus "stainless steel pan"). Chiyoh, herself, is somebody we know to be quite cold, rigid, and strong in her pursuit. She comes from a modest background (handmaiden to Lady Murasaki, lives in what appears to be a very self-sufficient and basic manner, lacks much apparent extravagance. Yet simultaneously, there is "silver", a much softer, malleable metal that's associated with a much richer lifestyle and is more refined. Chiyoh, herself, evokes a very clear sense of refinement below the surface. We can see in the cellar for the alleged murderer of Mischa that she has a refined taste of food, and (one presumes due to her association with Lecter) a similarly good selection of wine which is associated with a very upscale lifestyle.
Chiyoh, unlike many others in the show, has a great 'balance' in how she lives. Unlike DuMaurier, Margot, Mason, Alana, and many other of the show's characters who have a high class sense of taste, who live in modern homes, with modern lifestyles, and clearly rely upon wealth a great deal, Chiyoh is content with appreciating and balancing simplicity (iron) and a refined taste (silver); she perhaps lives the most reasonable lifestyle of all of those who've encountered Lecter in terms of balancing the cost of their lifestyle, because while yes, she does have a lot, and has exquisite taste, she doesn't flaunt it and appears to live a fairly simple life which she has refined to a great precision (even the way she kills, with a sniper, is suggestive of this).
Beyond the wealth, we could look a bit more at the concept of her in relation to being a victim of Dr. Lecter, and her malleability. Will Graham is extremely malleable, he can place himself in the mind of others with a great deal of accuracy and can understand there viewpoint. While he's very clever, and an excellent companion of Dr. Lecter's, this malleability and cleverness presents an issue to both of them in how it makes him very unpredictable and unstable. While Lecter very much enjoys and relishes this, and makes Will one of his closest friends, it's also something that makes their friendship one of escalation. His plan adapts continuously based upon what's happening, and he's a treasured friend of Lecter's. DuMaurier, on the otherhand, is somebody very, very rigid. She too is very clever, and also can analyse other's viewpoints well, but lives a strict lifestyle and is unwilling to take what she thinks to be very 'direct' actions. She is very much (in her eyes) an observer. She always has a plan, thinks ahead, and sticks to this plan (her routine this season, for example, was very rigid), but rarely carries out any actions herself even if she is willing to manipulate them as required. It helps in that she also has quite a cold, unflinching (professionally, anyway) personality. While she struggled maintaining her sense of self with Lecter, and it did waiver a few times, we as a viewer never really saw it dissipate. She is one who has interacted with Lecter, yet managed to maintain a very good balance of her traits. She adapts to situations, and shifts her plans, yet still sticks to a general idea and knows where she is going. The sensation that her interactive with Lecter was a positive one is also implicit in both of their traits (a speculative idea that Chiyoh teaching Lecter hunting prowess, Lecter teaching Chiyoh in manipulation and more refined mannerisms perhaps).
Of course, the metaphorical reading can be taken in many ways, it has a very broad range as it just relies upon the balance in her character in light of this episode. Nothing is solid, and nothing necessarily has to relate to another aspect, but I've supplied some suggestions on where an interpretation of the line may be taken. It is, I feel, the literal meaning of the line, that she is a very stable element in Dr. Lecter's life and is an individual who can always be counted on (her motivations, and otherwise), that is probably meant to be clearly taken from it if nothing else.