I haven't, but I cannot begin to fathom how that could possibly work.
How can the receiving router possibly resolve an IPv4 next-hop address
to an destination Ethernet MAC address, if the interface facing the IX
does not have any IPv4 addresses assigned?
A router only needs to know the l2 address to forward a frame. So there would be no ipv4 address on the transitory network just an ipv6 that resolves to a Mac address.
Kind of like ipv4 unnumbered but with ipv6 addresses on the segment. Multiprotocol bgp would be used for the next hops.