Oxygen isotopic ratios are paleo-temperature-dependent,
but I don't remember if oxygen is part of the protein structure.
Of course it is. :-)
But the temperature in question would be that of the water used to make the
amino acids! A carnivore will hardly make any amino acids de novo itself.
Even herbivores are limited -- vertebrates in general can't make any lysine
(so much for Jurassic Park). Plants make amino acids.
Oxygen isotopes are, however, supposed to be somehow fractionated when they
are used to build bone minerals. Investigations of bone minerals have
started over 10 years ago.