Thanks for this input.
1. Donation must be a burn, not a transfer, otherwise there will be wash transfers. The simplest burn to make is to run some PoW, so for new users that have no reputation I don't think there is a better costly signal than publishing a PoW event. 
2. I agree that local trust score is the right solution (I think people refer to it WoT, but it's vague), and we're experimenting with applying TrustNet (https://cblgh.org/trustnet/) on the client so that users wouldn't have to rely on our opaque global trust rank.