Specific profile fields presented for completion

Issue Summary

I had this idea some time ago, and this module is very close to it.

It would be very nice to have the simplest registration process, but to have a mechanism like this module to enforce the completion of some profile fields when the user enters in different parts of the site.

The site gains a lot of usability from this.
For example, imagine that when an user wants to see a list of pictures of other users, a block would pop up asking for his picture.
Or when an user wants to vote - and his age and gender are required for statistics - a block would pop up asking for his gender and age.

In this way, users will complete all their profiles in time, not being bugged by a long and boring list of profile fields to be completed.

What do you say?

PS. I guess a different approach may be needed, because an URL should have associated the fields the administrator considers to be necessary for that URL (the specific part of the site)