It seems as though avatars are not affected by occlusion culling. It seems like it would be a great benefit for maps which are broken up into distinct rooms to be able to stop wasting draw calls and physics calculations on avatars that a simple check would guarantee cannot be seen?

If it isn't possible to use Unity's native occlusion culling this way for some reason, maybe an even better idea would just be to give the ability for Udon scripts to decide for each user, which other users should appear to them as "culled diamonds"? (As in, in a script that would purely run locally, with each user's machine making separate decisions about how the other users will appear.) So a script could detect that the local user is standing in room A, and automatically diamondize all the avatars in room B, until the user walks into another room. (This status would of course be "or"ed with whether an avatar was already diamondized for whatever automatic reasons are already set in that user's settings.)