Is the spar cap on the Duo Discus bonded to the inner skin or the
outer skin? If it is bonded to the outer skin, it would not have the
foam to go through, which should ease the problem considerably.

Modern ultrasonic non-destructive evaluation (NDE) should have no
problem inspecting the joints in question. Rolls-Royce has developed
standoff NDE equipment that must contend with a 20 mm air gap. They
got excellent results detecting defects in carbon fiber samples:

Advances in air coupled NDE for rapid scanning applications
Farlow, R.; Kelly, S.P.; Hayward, G.;
Ultrasonics Symposium, 1994. Proceedings., 1994 IEEE , Volume: 2 , 1-4
Nov. 1994
Page(s): 1099 -1102 vol.2

Applications of through-air ultrasound for rapid NDE scanning in the
aerospace industry
Kelly, S.P.; Farlow, R.; Hayward, G.;
Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions
on , Volume: 43 Issue: 4 , July 1996

S-H is just talking to the wrong people.

Tom