> But isn't it possible to have a router convert from an IPv6 address
> with provider bits to an IPv4 address by simply stripping all but the

You can't strip a non-zero part of IPv6 address, as this would cause
transport layer pseudo-header checksum to fail.

Unless you recalulate the checksum. There are products out now
that do similar things (like map a large set of private ip addresses to
a single class C).