I'm an experienced PHP developer and I'm willing to implement this myself if there's no similar mod already, but I want to know if hacking the vB code to implement this kind of functionallity won't bring problems on future upgrades, or if there's a "clean" way to do it that won't interfere with them and allows us to keep the board always at the last version.
That's perfect, if you are an experienced PHP developer. I have not seen a similar modification available, but this should be fairly easy. It sounds like your run-of-the-mill insert/view/delete kind of thing. A modification that has its own PHP files and tables should be able to survive the any upgrades to the 3.x series. However, this may not hold true for vBulletin 4 (as a programmer, you should understand why).