It's been my experience that most players are totally supportive of donating to a worthy (Cause)/(Group) with no monetary prize expectations as long as the players know *up front* what their entry fees are going toward. This has known to be problematic in the past at some events where the prize fund did not match the entries because it was not explained clearly up front what was happening.
It's not like pinball is super lucrative anyway, but when people have expectations that are not met, even when talking about a little bit of money it can leave a bad taste.
By stating exactly what you are doing upfront, the players who are not in favor of such distributions can opt to not participate. Some people really do care about the money that can be earned, and that's fine. Almost all IFPA tourneys have an entry fee followed by a potential earnings distribution. Some players are rarely in the money anyway and they feel like donators nonetheless
Again, I feel the key is to simply say up front to all parties what is going on. Tell the players that the entry fee is X dollars and that N% of the entry fee will be donated to (Group).
Similarly let (Group) know that they will receive N% of the funds raised. With a fundraiser, there is no rule how much needs to be donated. It is free money for them. Obviously it is your/The Pinball Dudes call.
I suspect your typical players will still show up, and some may even kick in a couple more bucks to boot.
Just my opinion of course