Think back to Christmas day a couple of years back when a 172 hit power-lines through his own stupidity and the pilot subsequently died in the post impact crash and burn.
A local charity had recently raffled a flight with the crash pilot. Makes you wonder if the whole charity thing is a good idea.
For what a basic joy flight in a 172 costs with a reputable operator versus the income from the raffle or auction, why would you bother with the hassle?
You would be better off approaching a reputable charter operator or flying school and asking for mates rates, or even better asking if they would donate the flight.