No legal expert, and you've already got a very full explanation from StephenH...

My $.02...

The best rides that I've been on are ones where you are handed a cue sheet, maybe an identifying number, and told you are on your own. Suggest a couple local flavor stops along the way, and a decent watering hole at the end. No timing, no road markings, no sag.

Getting a couple dozen experienced riders together where each puts in $20 or so, with $10 returned in raffle style prizes (maybe gift certs from your LBS), should generate some $'s for your charity. This is a good way to start small and get the ride part of the organizing under your belt before doing anything bigger. Not enough money that way to create "expectations", but some commitment from the riders so that they want to stick around at the end for a prize, beer, and BS'ing. Have whoever holds the $'s gets a waiver signed from each rider, and a representative from the charity shows at the post ride location to accept the $'s. Make sure everyone knows the $'s are liquidated by the end of it, so they leave wanting to do it again.