I think the problem is that "payment required" is an issue that's better dealt with in a layer higher up the protocol stack.
One can simply respond with the appropriate redirection code to a login/signup page (or whatever) when the client attempts to access a page where "payment is require" (just like any other authentication/authorization issue).
One can simply respond with the appropriate redirection code to a login/signup page (or whatever) when the client attempts to access a page where "payment is require" (just like any other authentication/authorization issue).