Don't get me wrong, I love customization options like this, and if they could get it to work I think it would be awesome. Unfortunately the game just doesn't really support this sort of thing. I would guess this is also why we can't do things like name our looms. The game just doesn't really support it. Maybe they'll be able to do more stuff like this in MB2, but for now it's just not feasible.
As far as my limited technical understanding of how the items work, the items have stats (including name, damage, speed, etc) linked to their type, not to the individual reference that the game creates on spawning you. So if, for example, you have one person that gives +1 damage to their longsword, and another that gives +1 speed, the devs would need to make 2 separate types of longswords to support this. With the number of items in game, combined with 3 heirloom levels, that means thousands of new item types would have to be made.