Hey Sam,
I'm a climber and backcountry skier as well. I'm heading to the Methow this wknd or I'd help you on Sat. Couple things to note: You will need to be towed in the whole way. No sled can carry 3 guys and the gear you'll need to climb Rainier. Towing is pretty mellow if you are a decent skier, provided the snow is all packed out (which it should be at that elevation right now). Use about 20-30' of old climbing rope for each guy with one end tied to the bumper of the sled and the other end girth hitched around the center of your poles. That way you can use your poles like a waterski tow handle. It's also not a bad idea to stagger your rope lengths far enough apart to avoid crowding if the trail narrows. There is no motorized snopark at the Crystal Blvd turnoff, so whoever tows you in will need to figure out how to get their sled in there. I don't think you can park anywhere on the highway around that spot, but there is a non-motorized lot right at the turn. Not sure whether you would get ticketed for parking a sled trailer there though. Your best bet may be to try to find somebody on Sat morning at the FS 70 snopark near Greenwater, which is a pretty popular snomo spot. Bring your own tow ropes, some beers and some cash, and you might just get lucky. Be safe up there!