Even if they did offer, I don't know if Larian would want to signal to customers of future games that they're willing to put them on a subscription service just 3 months after initial full release. Not counting Early Access, which would be a different thing.
Could see an Early Access model working with a sub service for games in-development depending on the tier, though. But that's just for games in general, not specific to Larian.
How would this reinforce that take?
Again, it's not even so much about BG3 in Game Pass so much as the signal that would give to potential customers of future Larian titles. The game just had its full release on PC only three months ago, on PS5 only a month ago. People new to the studio's games would see BG3 immediately going into Game Pass as a sign that, well, they can just wait a few months for future games to go in sub services and not bother buying them at launch. And I think that would have an effect (to some degree) on customers on other platforms, not just Xbox.
Meanwhile it also creates problems for Microsoft, since it just further trains their customers to not buy games at launch and wait for them in the service instead. This, when they want to shift to pushing B2P sales, and a reason why they immediately said "no" to COD in Game Pass this year even though that acquisition is completed. They probably know better than anyone how erosive the subscription model is on B2P sales, since they have faced the effects of it more than probably almost any other publisher in the industry.