I referred to your idea in https://www.mail-archive.com/bitcoin-development%40lists.sourceforge.net/msg04076.html which is indeed not the proposal itself.

Right, gotcha. Had forgotten about that.

Indeed there is another signature, which is to authenticate the payment delegation. If you take it into account in the signature count, then your proposal has three signatures.

Yes, I see now, you are right. A mandate type system is probably simpler indeed.

So what now? To be honest my next priority with BIP70 was to formalise the extensions process, I've been dragging my feet over that because I'm working on other things. And then after that to knock some heads together over at BitPay/Coinbase and get them to put useful text in the memo field instead of random numbers. Baby steps ....