I agree that this is going to be a good test for the H-D PA, being brand new and all. That being said, I assume H-D has done extensive torture testing. But, I don't know about all the other bikes being non-standard. As far as engines anyway. Of course the seats, suspensions, and such have been changed out for aftermarket.
I have absolutely no experience with IBRs, but the ST1300 was ridden day in, day out. . . . mostly shine, but a little rain tossed in. And, the only things that were replaced were gas, tires, oil, and brake pads. No mechanical failures, with the exception of the left fork seal every couple of years. The RT on the other hand. . . .
My point is, the bikes don't seem like they would be the weak link. It has to be the rider(s). I have no doubt, that I could take my bike as-is, and complete the rally without a failure. . . . if I wasn't a weak link.