Unless they're so secret and hidden..they hide them on a different website O_o
I think it'd be cool, but two things would be wrong:
1) You'd have to know the game that it's for, or they may never be achieved.
2) Once one person gets it, it might not be so secret. Even if when it shows up on a person's quest list as "Secret Quest 4" or whatever, who's to say they won't tell what it is? Secrecy lost.