Can (part of) the fuel in your tanks be considered "property carried for compensation or hire"?
N707MQ actually has a special type of experimental certificate, according to its registration it is a "Market Survey" aircraft as you note.

The FAA defines this as 


  Market surveys:  to conduct market surveys, sales demonstrations, and
  customer crew training for U.S. manufacturers of aircraft or engines.


This does not really provide us any special way to use the aircraft for commercial purpose but is worth noting. 

Either way as you state 91.319 pretty clearly prohibits this. Interestingly enough 91.905 even prevents you from getting an FAA waiver to override 91.319 (as its not on the list of things you can have waved).

However 91.319 sub section H may provide us with a solution. 


  (h) The FAA may issue deviation authority providing relief from the
  provisions of paragraph (a) of this section for the purpose of
  conducting flight training. The FAA will issue this deviation
  authority as a letter of deviation authority.


As far as I know these tankers are primarily used in our own airspace for training missions which would fall under "conducting flight training".

This article is pretty interesting and covers a similar scenario for using experimental for commercial purposes.