I'm already on way with developing this hack, it's not very hard at all. Your suggestion makes perfect sense, but I also plan to have a post count checker, and only allow my members with at least 300 post count to use this hack (that number ofcourse can be set by you).
Having a members page be checked each time they make a change, whether small or large (ie. changing a font size) would get really annoying for the member and the moderators. I think just allowing the custompage for only those with higher post counts is better, since those who have been on the board longer know the rules and will act more responsible..(hopefully).