Hmm, I don't think there's any other way to do it for this case that isn't supported by a server responding or updating something when required, since there's no magic way to expose something, detect usage, and expose something new next time without requiring payer interactivity. At least for the xpub case where you want to generate a new address when someone uses the previous one. I was thinking sometime ago about "payment channels" and dh, so you can set up something more or less static that "changes" but with people you previously set up that dh. I mean you could calculate all shared secrets for all your contacts and assign a different xpub to each one. But it seems quite convoluted.