Children typically dislike exploration activities in the middle of fluency activities, and for a very good reason - the two interfere with one another! If the kids just learned a multiple-step algorithm and are trying to memorize it better, exploring alternatives will mess them up.
There are quite a few existing collections, in books and sites, unfortunately NOT aggregated in one place yet, of lesson plans devoted to children inventing notation. John van de Walle had written about it, for example. "Living Math" community, led by Julie Brennan, has a lot of discussions about this. I do this activity routinely, with all my students in most of the topics. When this is going on, students LOVE to look at multiple historical or modern notations, which we usually do after they've invented their own. This way, they see themselves as a part of the long continuum of math creators.
For the purposes of such an activity, my wish list is:
- a place like your wiki available (check!)
- a place just like that, but for student- and teacher-invented notations
- a cross-linked depository of lessons/activities using the above census items (somewhat like Joel's
http://geogebramath.org/lms/nav/index.jsp); that is, aggregation of links to activities where each notation is used on notations' pages, and links back to the notation census from activities' pages
- a way of commenting back and forth with people who are contributing activities (like blog comments)
The next step toward my wish list is to start a sister wiki for student notation. Your work is so inspiring, Paul!