I thought as long as you had actions on additional crossbars, it would include it in the RB toggle. If not, there should be controller settings that determine which will show up.
I'll have to try it when I got home. Right now I only have Sprint, Mount, and the 1-2-3-4 Marks set to crossbar 3, and to access it I have to hold the R Bumper and hit A. The toggle may very well extend automatically when Action commands are added.
Also, another thought just occurred to me. What's the range on Holmong? If it's far enough, it might actually be useful as a means of locking down Add's with AoE attacks, before they run off to chase a healer.
Edit: For example so, during the ADS fight in Coil 1. Use Holmong right as the first ADS ADD spawns, locking it in place for several seconds, out of range of most of its attacks. It'd give you a couple safe seconds to throw a couple Tomo's, maybe hit a CD, and alert any support DPS that the first ADD has spawned.
It'd only work if it's range was just as good as Voke's and Tomohawks though.
Edit 2: Looked it up, and it's a close range attack. Shame, since being able to lock an enemies movement from a distance would actually be extremely useful.