Looking at this (Apple AirTag Reverse Engineering - Adam Catley) post, the airtag can be trimmed down just small enough to fit in an expansion card. Not got one myself to try it out on, but theoretically doable.
Also looks like someone else on the forums did it here (AirTag Expansion Card)
If you put it in a 3d printed enclosure, I imagine that would help with the loss of signal compared to putting it in the bay