I've argued strongly against NAT, but he's one of those people who seem
to be willing to accept arbitrary amounts of pain ("we don't need to
use [protocols that put IP addresses in payload]", "timeouts aren't
a problem"). I'm now pointing him at some relevant RFCs. My question
for the list is is there a web page or other document anywhere that
comprehensively states the case against NAT?
This organization makes its opinions known through not only standards but RFCs that have been reviewed by IETF working groups, the community, and the IESG. RFC 2663 is the one you want. See in particular scaling issues, multihoming, DNS, and IPsec, just to name a few.
Eliot