BIP70 supports the wallet returning a "refund address" along with the signed transaction to the vendor.
I would assume it would be possible for the flow to work like this:
1. User clicks BIP70 enabled link.
2. Site sends signed request to a null output. "00" or something arbitrary.
3. This value should indicate that the wallet client just return an empty reply (no transaction) with just a return address.
It's primitive, but rather than try to create a new BIP, using existing BIPs that can provide the functionality would be easier.
I had considered 'hacking'
BIP70 as you describe to specify a return address. And it does conceptually fit with the abstract of BIP70: "a protocol for communication between a merchant and their customer".
The main barrier is wallets that implement BIP70 as currently specified will not work with this new scheme. Should BIP70 be revised/extended to include this new functionality the way BIP71-73 do?
edit: The
extensibility section of BIP70 looks like it could be utilized for this purpose, but it really depends on whether wallet developers are willing to implement proposed changes.