The problem with doing a client for this is lack of scale. Assuming there's 100k active users, 50% run iOS/Mac, probably < 10% of those would pay for an app. I'm thinking ~5k copies which sucks compared to the effort required.
@pth I'd bet there are more than 100K active users; 1 in 10 of total users seems conservative. But IIRC @Gargron said there's no tracking, so it's probably hard to be more certain. I'd bet @Unranged is right about this group self-selecting for willingness to pay, though.