Mystery Characters - In Between Time

Another issue with Mystery Powers, is the "in between time." In a typical game between the actual sessions, there is the "in between time" that is assumed to pass, without actually role playing it.
Now, in a game of this sort, I would expect people to say that they are 'testing their powers' trying new things etc.. virtually every moment that they are not engaged in adventures.
So, how long can 'mystery powers' be kept mysteries?

Possible solutions.
1. Let people use 'off-line' communication to do practice and give them feedback. This would reward postings to a game blog, or listserv with tangible improvements (or revelation) of their abilities.
People who want mystery can keep it, by not doing anything, or posting other activities apart from practice.

2. Run the games 'back-to-back' in game-time, so there is no down-time without the GM being there. They can practice, but you can interupt the practice with adventures.

3. Give them one or two powers, and lots of experience points, so they can 'build' their character up from near scratch. However, make them submit their new characters and apply a 'line-item-veto' to any powers you don't want them to have yet.
This looks too much like normal character generation to me...