I haven't read the IDNA draft either, but I'll accept your analysis
and quibble a little with the characterization of OPES as acting only
on "valid application messages". One value of proxies is the ability
to deal with commonly malformed requests and responses - things that
were not well described in the application specification and lead to
unneccessary loss of interoperability. Being that this kind of fixup
is a reasonable proxy function, it should also be a valid OPES
function. So, I'd think IDNA may well be an OPES scenario.
Hilarie