Update to this, kind of as expected- Enve was immensely helpful and could provide plenty of documentation around the stem interface. FSA, basically useless from who I could get through to. Luckily the FSA geometry is really straightforward- my local shop had a spare top cap so I was able to draw it up in less than an hour.
Enve asked I didn’t share the files around, so anyone reading this now/in the future should just contact them directly and I expect you’ll have as easy a time as myself!