Kathome:
We also ride (well I just sit there and look pretty) and have participated in your standard dice /poker runs. These have always done well in our area for fund raising. Lately though, instead of just doing 50/50 raffles, some have had auctions before the prizes are handed out.
I really think that some biker themed "Bow Tuck" totes would create interest in bidding because the gals can always get the guys to buy something for them at a charity event.
We have a Goldwing and pull a trailer so we have tons of room to haul stuff. Some people just have their saddlebags so it needs to be something that will fit in there or make arrangements for shipping.
If you have people that are "ate up with" Harley items, you could get handerchiefs and use them as fabric in purses / totes / wall hangings / aprons / napkins/ placemats, etc. Just be careful of any possible infringements by using the material (don't think a charity auction is a problem though).
Good luck with fund raising and thank you for the work you and your friends do. It takes special people to provide special needs and it sounds like you are definately one of them.