FWIW, there _was_ discussion in WREC of the hazards of transparent web
caching. I dug up an old e-mail, describing the hazards of transparent
web caching which I summarized at the time, when WREC was forming.
A copy of the note, admittedly very rough (just an outline, and a very
rough one at that) is at:
http://www.isi.edu/touch/pubs/hazards-outline.txt
I would be glad to host further discussion on the WREC maillist as to
how to augment the list and flesh it out to a full I-D before the next
IETF, if there is sufficient interest.
Joe