I can't see any problems with a donation button? AmigaSYS and Amikitt both use donation buttons and no one is bothered? They too contain other peoples work.
It's completely voluntary if anyone wants to donate and I'm sure the launcher itself is 100% free without donation nagging screens popping up etc.
Plus Synchro has put in a lot of his own time getting this all together for us. The donation could be just to cover bandwidth and hosting costs for all we know! If people want to thank him for that with a donation, that's their business.