You can’t just do that with a HTTP proxy, because HTTP requests also involve DNS lookups. So you’d need to have at least a DNS server running and you’d need to alter the domain you wish to visit with some other domain, which would then point to this DNS server. Comparable to what SixXS was doing. But I don’t know of any service that provides this.
I’m testing with random anonymous http proxies, and it looks like domain lookups both bypass both my local host file, and my ISP dns (i checked with some banned domains).
So i wouldn’t understand why a proxy couldn’t fetch content under ipv6 by default, and relay to a ipv4 client?