2) Use DNAME, RFC 2672. Good luck.
(http://www.isc.org/index.pl?/pubs/tn/index.pl?tn=isc-tn-2002-1.html)
3) Use RFC 2317. "I encourage my competitors to operate this way."
Note: DNAME is equivalent to RFC 2317. In both cases this
will break the customers expectation that they can just use
x.y.z.in-addr.arpa for the PTR records.
Note for reliable local reverse lookups when the external
link is down they will need to slave the ISP's x.y.z.in-addr.arpa
zone and well as manage the zone the CNAME / DNAME maps to.
Mark