It can't even address a 128 bit endpoint, so nothing would happen.

Sure it can, the DNS server returns the A record if your client doesn't understand Ax. It just won't work.

Honestly, this backwards compatibility thing seems even worse than IPv6 because it would be so confusing. At least IPv6 is distinctive on the network.

	▲	lxgr an hour ago \| parent [-]
		The question was about forwarding as I understand it, not address resolution, and there simply won't be any forwarding, since the 32 bit only sending host won't be able to address the 128 bit receiving one.