Gosh, Ben, you are so technical at the moment!!!
Really, I'm just wondering if anybody in the "real world" has ever had problems by replacing a C tire with a D tire on a wheel stamped for 50 psi.  I have been running my Marathon 205/75R15 C's at 55 psi for the last 4-1/2 yrs with no problems.  Really!!  Marathons!!!  And the same with our last trailer for 6 yrs (Marathons again!!!).
I just started thinking aboot this upon first seeing this thread and after talking with my son (Ford Service Manager) who mentioned that a lot of Ford PU owners who have steel wheels have been damaging wheels when they go from "P" tires to "LT".  Mostly (for his shop) commercial trucks, and obviously a great change to tire pressure!!  Door sticker psi for a P, to 70-80 (or more!!) with an LT.
Going from 50 psi wheel spec up to say 55-58 with a D tire really shouldn't put the same kind of forces on the wheel as say the 65 psi max of a D.
Agin, just thinkin!!!!!