(Doug) wrote
I make the arguement that
the tailwheel pilot is PIC for the entire flight, and the instructor
is accepting the IFR clearance.

Then the tailwheel pilot is in violation of 14CFR61.3(e):

14CFR61.3(e) Instrument rating. No person may act as pilot in command
of a civil aircraft under IFR or in weather conditions less than the
minimums prescribed for VFR flight unless that person holds:
(1) The appropriate aircraft category, class, type (if required), and
instrument rating on that person's pilot certificate for any airplane,
helicopter, or powered-lift being flown;

Michael