Do you - or anyone - think there will be any spots left over after on-site registration? Did any spots go online for IM USA last year?

Yes, but so has IMC and this year they opened registration 2x online after onsite. I would this year of any, the chance of having online registration for USA would be better.

There were NO spots for IMLP offered online last year. It was an onsite closeout, except for Community fund slots, of course. Britt makes a good point..the economy being what it is...there might be a few slots available online.
My wife and I and a friend volunteered last year and I've written about it several times here...if you do a search of IMLP Volunteer, I'm sure you'll what I wrote about our experience. We were at the Boat House aid station which is right next to special needs. Usually they get crews together on Saturday to explain everything...They have it down to a science including what to do and not do. Follow their lead. You'll find some riders chatty and others that will be quite surly if things aren't going well. Tread lightly. Some folks don't like to hear "One more lap!" others do. Good luck figuring that out! Be aware of folks that stop for stuff. Some are oblivious to the riders behind them. Encourage them to stop and start out safely. Be careful yourself stepping off the curb to hand someone a bag...I have seen folks get hit. Other than that, just be happy and smile and be of good cheer. I'll be there this year...my fourth IMLP...My wife and Beads, I believe, will be at the Boat House Aid station.
Oh...you'll also be right next to the Lake Placid Brewing Company, which has a beer tent set up...
See Ya there .. .(Oh, I'm getting excited!)

Sorry, it's just a relevant topic. I just can't justify travelling up there to "volunteer" knowing fully well I'm not that altruistic and I'm just doing it to get a spot for the next year. I see you are already in NY. The closest IM to Dallas is 800+ away in Florida. If I volunteered I feel I'd be contributing to the problem of excess demand, thus reinforcing IM NA's over-hyped entry policy which breeds a sense of exclusivity and accomplishment without actually doing anything. You got in. Big whoop. (Not you, WillbIM1Day, the general "you.")

You, though, should be commended for volunteering for a less-than glamorous chore (even if you have alterior motives. But who am I to question your motives?) You could've gotten off easy by being a wetsuit stripper, finished by 9am. Or, you could've been a "sunscreen slatherer," but speaking from experience, I prefer my industrial grade SPF30 to be applied by volunteering high school cheerleading squads. Of course, you could've been a 'finish line cathcer,' but those are probably more coveted spots. I don't know why. Finishers are often layared in sweat, pee, vomit, and poop (in that order).

But to your OP, you volunteered to be stuck out in the-middle-of-nowhere, miles away from the start/finish. If I were doing special needs bags, I would expect a lot of over-stressed, self-important Type As to yell at you for not finding their bag fast enough: "I need bag 1452 right now! Can't you find it in the back of that giant box truck mixed in with all of the other identically colored and sized bags?!" You know they need their mushy PB&Js and spolied protein drink immediately or they'll lose their Kona slot. Riiiiight. I'd love to see someone dressed as Didi Senf (El Diablo). Ironman needs more schtick and less stress.

I volunteered at the special needs station last year at Ironman Wisconsin. The "training" ammounted to: 1) Pick a range of bags to stand in front of (they are all ordered by number) 2) Don't help the atheletes with bike repairs, etc... 3) Here is your free t-shirt.

It was pretty low stress and a lot of fun. Almost all pro/elite competitors didn't stop for their special needs bags. Everyone who stopped was very thankful for the volunteer help and at least half of the people who stopped were no particular hurry. Some people would stop, sit down on the curb and eat their sandwich, etc...