I was able to do the doc wong CPR certification with special emphasis on motorcycles and downed rider scenarios. There was a lot of great info, but I'll try to remember some of the best stuff:
-First thing is to control the traffic. If you've got flares, use them. Get a bike or bikes and park them in the middle of the lane where cars have time to see them. So if there's a downed bike in the middle of a curve, get around the curve to where there's some straight line visibility and leave a person, bike, flare or something to slow oncoming traffic. This is the FIRST thing you do, and you do it in both directions. Nothing worse than having a sports car in the middle of a four wheel drift interupt a crash scene.
-Say a downed rider critically needs to be moved. He's not breathing and is off the road in a bad / non flat location, or is in the middle of heavy traffic, etc. He's wearing a helmet, and you're not sure about neck / spine injuries. Also, you've had CPR training and are willing to risk the liability in moving him... as gently as possible, straighten the head/helmet and standing with your feet facing his shoulders, gather a handfull of his/her jacket somewhere between the armpit and shoulder area in each hand, and with his/her helmet cradled between your forearms, drag the downed rider to a safe and level area (where you can start CPR if needed). The idea is to risk further spinal damage only if there is a more serious problem (not breathing halfway down a cliff, truck going to run him over, etc).
-Once you've got traffic under control, downed rider in a safe level location, you've got to decide whether to start CPR. The part I think I was most surprised about is this: if you start CPR, you don't stop until relieved. This can take a half hour or more of serious work, and if they need CPR they're in serious trouble and probably won't make it anyway. It will be exhausting, and if you're out of shape, you may not be able to keep it up very long. So you have to decide. Don't discount the liability issue either: if you don't have a choice and have to act to save a life, you'll probably be allright legally, but if your actions could possibly have severed someone's spinal cord, you could also be shelling out serious bucks to a lawyer in consequence.
Don't forget about AIDS and bleeding mouths either. After the course I took, I carry a mouth damm in a plastic case (about $8 from Galls.com) for that purpose. It's like the plastic cup shape you see go over peoples mouths when they're given oxygen, with a little valve that keeps any blood from getting to you. (Cops carry these as well, and if you have to move your well marked plastic case from under your seat to get your registration, the cop will see it and it may influence his decision to give you a ticket. Or so I was told...)
-If they don't have a pulse or aren't breathing, or both, and you've decided to perform CPR, you don't have a choice - you have to remove the helmet. You'll want the help of at least two other people (ideally) for this. You'll probably want to kneel at the downed rider's head and very gently undo the chin strap. It's a good idea to keep the chin tilted back a bit. Gently get your hands and fingers supporting the head, probably with some fingers up inside the helmet. Direct your first helper to do one thing only: hold the two chin strap sections away from the head, pulling a little to open the helmet more. Your second helper will slowly wiggle the helmet off while you keep the head and neck as inline as possible. Once it's off, you'll need to get the jacket open or off, and any other layers as well. For this you carry EMT scissors (also a few bucks at Galls or any medical supply house), and you don't hesitate in cutting away clothes. If their life is ticking away, don't worry about a nice sweater, etc as they may have arm injuries and in any case, removing a top without twisting the spine is next to impossible.
-Now you're in regular CPR territory. 11 compressions to 2 breaths (if I remember that right, pretty sure...). I also bought a package of triangular bandages (40x40x56") and keep a few on the bike. Good for slings, pressure compresses, and any padding or bandage needs. Also don't forget that often during CPR, the unconscious person will vomit and, if you do your compressions right, you may crack the sternum or some ribs. Helps to be ready for such things.