Can you explain why you don't like the idea of an progress callback or
something like that?
I didn't say I don't like the idea ;-) I was just curious (and lazy to implement the partial access), that's all. In the end, I'll probably use the solution I've seen in qed -- a small progress bar while background is inaccessible, in that way user knows something is happening.
Thank you all for the ideas / notes.