My guess would be that we're not allowed, but I don't know. Have the excommunications been lifted on every hand or what is the current situation? I mean other than a crying shame, that is.
"My understanding is that it's part of the discipline of the RCs that we don't communicate with separated brethren."
As a general proposition that's correct. Do you know why?
As for Orthodoxy, generally USCB says you can commune with us; we don't allow it. Do either of you know why?
Orthodoxy does not allow us to receive communion in any non-Orthodox church or ecclesial group save under certain circumstances with the Oriental Orthodox.