I don't see the point of this; having 1000/1000 on a game is the same thing as what you're describing. If I check out your gamercard, I can see which games you've got 1000/1000 on quite easily; there's no need to examine every individual game and check for that extra achievement you're describing.
If MS wanted to copy the e-peen factor of platinum trophies - where the're clearly visible at a glance, and don't require someone to browse your game list - they could just add an additional score besides gamerscore. That score would track how many games you've collected all achievements for. Call it "completed" or "flawless" or whatever. When it comes to internet forums, MyGamerCard.net could implement something like that without Microsoft doing anything.
