I messaged Hope last year in Winter asking whether the RX4's are safe for loaded tandem touring, and their reply was "the RX4s are a powerful 4 piston caliper, with larger rotors they would be suitable for tandem use".
They did not go into safety or legal aspects. I guess that they cannot take the legal risks of doing that, but based on that reply it seems that they are confident in the capabilities of the RX4's.
The only other hydro brakes that I considered for heavy loaded touring were the Zee's or Saint's.
Officially they are not compatible with Shimano STI's, but I've seen 2 independent blog posts of people pairing them with the Ultegra STI and being satisfied. The larger body and finned pads should result in a higher heat tolerance.