Good friggin luck!
I currently work for a Part 135 operator, and can tell you getting jumpseat approval with a 121 carrier is about nil. Unless the company you work for has a specific WRITTEN agreement with the 121 carrier, the chances that you can get on are slim.
I used to enjoy jumpseat privledges with many carriers, written agreement or not, now since 9/11 , no one will touch 135 guys. I have heard of some exceptions, but I think there is some verbage from the FAA after 9/11 that basically says you have to be 121 to ride 121, and 135 to ride 135.
Granted its been a few months since I tried it, but I just gave up on trying until I hear there's been a change.
If anyone else out there has had better luck with specfic carriers or things have changed, please let me know.