Andy, Good job in supplying information.
I'm at a loss as to why the 137 feet of cable that is exposed to air instead of mortar would have a lower resistance as compared to the 422 feet of the cable buried in the mortar. If anything, that 137 feet would have a higher resistance because its operating temperature would be higher. (The air will not conduct the heat away from the cable as fast as the mortar.)
If your intention is to leave the 137 feet exposed to the air, have you checked with Maxxon to make sure this is ok? Other than the possibility of overheating, (which my personal opinion is that it is not an issue since we are talking about less than 3.4 watts per foot) the exposed cable could be accidently pulled on, etc. unless it is properly stowed in some manner.
At any rate, the only reason I can see for reducing the voltage would be if you were to cut off the 137 feet and connect the 422 feet only. In fact, that works out to be 181 volts. (422/559x240=181) Otherwise, as long as you get the ok from Maxxon to run the cable exposed in air, and the loose cable is properly stowed NOT coiled up, I don't see why you couldn't run it at the rated 240 volts.
Disclaimer-This is my non-professional opinion, you should get an opinion from the manufacturer and licensed electrician in your area before going ahead.