@Edent I'd be afraid that it would sharpen incentives to steal posts, impersonate, steal accounts, and maybe write divisive things.

I'm somewhat hopeful that some limits (rate-, account age-, ...-dependent most likely) on part of both involved instances could curb the post stealing and impersonation problems.

Can you think of a way to meaningfully experiment with that in a way that doesn't need to involve the potential-tipper's instance, only potential-tippee's?