A GitHub issue was already opened, see Preparing Exchanges for Grin 5.0 and issue.
They want a web service where mediators can verify payment disputes without running their own node. I actually wanted to make one myself but had difficulties with the wallet API, which I think are now resolved thanks to @renzokuken and @david. So once I manage to get a few things off my plate, I’ll give it a try.
Meanwhile if anybody else wants to pick it up, feel free to do so.