Hi, Craig. Sounds like a great trip. A few thoughts:

More calories, definitely. Even a big jar of peanut butter.

Tarp and light rope for quick windbreaks or the roof of a shelter? (A colour that is highly visible on snow would add signalling capability.)

Insulating pad between you and the cold, cold ground?

Folding saw for firewood?

Siphon tube for getting gas from your sled? (I assume you have oil injection; not sure if the MSR will run well on pre-mix.)

Signalling gear -- maybe add a signal mirror and a super-loud noismaker (like one of those hand-held air horns); maybe some flagging tape also?

Flashlight/headlamp?

Toilet paper?

Finally, is this kit duplicated for each snowmobiler? Maybe split up the PLB and the SatPhone between the two of you?

Have fun!