Is there a way to do that at present? It doesn't seem to respond to the lens settings.
No - sorry. It would be *really* confusing to have it project spherically in the viewport, wouldn't it?
I don't know why this would be an issue. Obviously somebody desiring this feature would just want to switch to a 180 degree view for this functionality, and then switch back. I could see this being very useful for previewing, particularly for those of us who pipe viewport output into a realtime compositing package for rapid previz.