I seem to recall there is another way to suppress CFMU checking, using the term "GAT" this came from DFC originally I believe.
Nobody seems to know what the purpose of this is but given the obvious meaning of "GAT" one would assume this is a deliberate feature of the system.
I've had ~ 300nm direct legs in Europe, at FL100-180, but they were within a single country. They way to get these is to ask, and keep asking.
In any case, ATC sometimes need reminding; they can sometimes forget about you and then you fly past the end of your "DCT" or "own nav" instruction and wonder if you should revert to the filed route. So, while reminding them of your existence you may as well ask for a DCT a few waypoints past.
In reality there appears to be so little traffic at these "nonpressurised GA" levels that I wonder why ATC don't do this more often. One can fly 500-800nm across Europe, in gin-clear weather, and not get visual with another plane.