Hi Peter,
Fabric (all versions) only uses Paramiko, so if it lacks host-based auth, then Fabric will as well.
I looked and there is an open ticket for adding the feature to Paramiko:
https://github.com/paramiko/paramiko/issues/316 - there seems to be a diff attached which I haven't reviewed yet (hasn't been a high priority as very few users seem to be requesting it).
Should you have the time to attempt adding it to your local copy of Paramiko, please comment on the ticket with the results - that would help move things along a bit.
Best,
Jeff