Well, there’s been a lot of discussion about this before, some of which you were involved in. Dave had some considerations here which I find relevant. Also Reuk here. I’m not convinced that a timer-based approach would scale well: it may end up causing more problems than it solves. I can’t say I’ve never experienced priority inversion from these things, but if it happened I’ve not noticed. Definitely not a simple issue though.
Back to the op’s, I meant to point that even if you can call setValueNotifyingHost() from the audio callback with no particular concern, it may be good to know that it won’t write automation. Not sure if that’s relevant for MIDI mapping, which I’ve never done. There are a number of threads about this, and there’s Daniel’s implementation in Plugin Gui Magic.