If you want to edit a part while it is isolated, why not just open the part and edit it by itself?
Edit: Sorry after reading your post again it sounds like you want to switch back and forth between isolated and not isolated while editing the part in context. So in that case you wouldn't want to open the part by itself. I can't help you there.
Jef, I agree with you completely. When I'm editing in context and I want to see only a few components that apply to my area of interest on the part, it would be very useful to be able to invoke the Isolate command.
I thought I'd put in an enhancement request for that, but it may have been at a previous job, so I don't have access to that.
I suggest you submit an ER for that functionality to be included.
If you receive an SPR number, post it here for others to add their vote.