Maybe this could be an option for server fees, but this is for iR0b0t to decide, because he's the one paying for the server fees atm. Purchasing games for dumping seems like a legal grey area though, because you use donations to add hashes and info to a database, but somehow these dumps also end up on people's HDD's. MAME has been doing succesful fundraisers though for purchasing and dumping rare games, but privately and not via Patreon.
Another thing is that dumping rare games doesn't have to be a loss-making enterprise. I have dumped plenty of rare games and either sold them on (also with profit) or used my legal right to return the games for a refund. I think most of the remaining dumpers have either found ways to get and dump games as cheap as possible, or they really buy the games for their own collection. Donation money should only be used for buying games and selling them again after dumping, and not to fund private collections.
And the final argument is that, even if we got $1000 per month for dumping Saturn games or whatever, we would need trustworthy dumpers with a proven track record and with a lot of spare time who are able to deliver these dumps in a certain timeframe. Most active dumpers just don't have the time to do a lot of dumps each week.