Sendel's email reply to me simply confirms their spec's, which appears to be shared by every other 14" and 15" trailer wheel I find. There may be a 14 or 15" wheel out there somewhere that can handle 60psi+ cold, but heck if I am spending anymore time looking!
Sendel's email;
For your question regarding air pressure it is important that you stick with our max psi of 60psi cold. When tires are run they develop heat and the pressures increase. If you have exceeded the pressure and then it increases it could possibly go out of parameters that the wheel was built and tested for.
Thanks and take care,
Kelly Austin
Tireco (Sendel and Forte wheels)