This sounds like it would be better as a MOD/Plugin. Should be rather simple to install once we have all of the hooks figured out for 3.2. 3.1 will be a big step in that direction. In any case it would end up being the same thing. You want this feature? Download it and place the files on your site...done (well that's the idea anyways).
I think you would need one or two extra tables, but that doesn't need to alter the original phpBB3 tables (perhaps a mod could require it). But just look around if you found a mod for that. Adding one or two mods doesn't make your board incompatible to new releases of phpBB.