This year the Welsh Ride Thing was done in aid of the Air Ambulance, so it was a charity ride just not a usual ride … it's spread over 3 days and 2 nights with riders having to carry everything they'll need plus they have to devise their own route and navigate.
It isn't a sponsered ride though, money is raised by each rider paying a small fee to take part and by a raffle with prizes kindly donated from the bike and outdoor world. I think if you want riders to get sponsorship then it needs to be something really interesting/exciting to to fire interest … raising money for a kids charity helps here too.
If you've a charity in mind then I'd contact them as they might have some good ideas, etc.