This can work.  I noted you specify how to use WebFist as the way to publish the JRD.  However, you don't need to do that unless your email provider does not support WebFinger.  I run my own WebFinger server and could just publish the JRD directly myself.  In fact, I do. You can query my WebFinger server like this:

$ curl https://packetizer.com/.well-known/webfinger?resource=paulej@packetizer.com

You'll see I have a couple of bitcoin-related entries in mine already (and I show one more below):


One of those is just a Bitcoin address.  The other two are Bitcoin payments URIs, though I suspect the latter is more along the lines of what you're expecting to use.  The URIs I use as the link relation types ("rel") are made up, since there is not standard specified for those.  However, I think those are reasonable ones to use and follow standard conventions.

I'm not terribly familiar with BIP70.  I can understand how you might publish a URI to which a payment is made, but how would you receive notifications of payment requests?  Does the wallet poll some location from time-to-time to see if there are requests?