Just looked them up in our jumpseat list and they are there so I know that they at least USED to offer jumpseats to 121 carriers (or mine anyway). Also there was a note there about a guaranteed return flight (nice touch). Are you trying to set up a new agreement with them or just get a ride yourself? If its the latter I would simply call reservations and ask for the operations number to get details. Do they go out of SEA?
I used to fly for them. They have 4 flights a day from Boeing field to the Islands. They have added carravan's this year, so your chances should be better on getting on. They used to send only C206/C207. They are a great bunch of guy's. From what I remember they had agreements with almost everyone. Also most guys would give you a ride regardless of who you worked for. It's that type of company. I used to take some of the pilot's on a quick tour of the Islands if time permitted. Never hurts to ask.
All of Kenmores scheduled flights depart from the south end of Lake Union (by downtown). They do honor several jumpseat agreements with various cariers. There are no scheduled flight from Seatac to the SanJuans any longer. West Isle flys out of Boeing Field and I think Bellingham, they also have jumpseat agreements with certain cariers