Regarding flights above 18,000 feet during flight test periods.
It's my opinion , that FAR 91.305---91.319 and FAA order 8130.2 F  do not explicitly prohibit a flight above 18,000 feet.  The intent of the order is clear that test flights during this phase will limit flight with in a designated area that will not be over densely populated areas/congested airways or create a hazard to any other air traffic or property/ personnel on the ground.
That said, I believe if the geographical area is satisfactory (sparsely populated) and if it is in an area that ATC would approve high altitude testing, --that it would be the basis for a DAR --with the concurrence of the local FAA MIDO/FSDO--to write this flight test area into the operating limitations.
I can see no reason why it would not be approved to flight test if all other requirements are met.  (Pitot static/altimeter/transponder tests/DME etc. etc.)

After all, Space One did it!

Strange how some government folks come from the "here's why you can't do it" perspective-- instead of what is the intent of this regulation- and here's how you can do it -and still be in compliance.

As an example-- The EAA went nuts several years ago trying to clampdown on several DAR's and ASI's who insisted that an A&P must sign off on the final inspection prior to issuing a airworthiness certificate.  They finally wrote it plainly into the A/C 20-27F "not required ."

Charlie K. (DAR)