Hmm, it's a good idea but I don't believe something like that is possible for me to do without requesting a code. And since zeta was announced, people have stopped coding for IF and are focusing on zeta even though no one has any idea when it's coming out.
I do know that in zeta I can make custom profile fields(you can see an example on our beta board:
http://beta.zetaboards.com/vix_katamari_beta/profile/3898/ ). You can see I added spots for psn, xbox live, and wii codes. i could technically add in what you want them but I wouldn't be able to make it appear in sigs, only in profiles.