position and rotation modifier is primarily for the general "position" functions of entities, generally you can see it work if you mess with prop_dynamic things that don't have anything physical, so physbones modifier doesn't work on them. although i'd suggest to use both physbones and position/rotation modifiers on one timeline for convenience, as spawning would freak out without these things.

currently smh doesn't support recording on separate bones, i was thinking to take a look and try to get something done to work with that, however i can't promise that i'll do it soon.

Pity. In some situations, this is convenient. I assumed it was impossible. I will be waiting for updates. Thanks for the answer)

I would really appreciate a feature like this, if its possible! :D