When my pinched nerve in my neck was really bad, I had to find a way to hold my ruler and still cut with my right arm. I placed my cutting board on the kitchen floor with all my stuff within reach. I used my large 15 inch square, arranged my fabric on the board and SAT on my 15 inch square to cut. The whole point is ....you have to be able to anchor the ruler in order to cut with a rotary cutter from a non-standing position.
For your situation, if you could anchored the ruler, you could cut sitting down. How about a piece of plywood that you could use carpenter wood clamps to hold the ruler? You could place it at a sitting height or even on your lap to cut.
I also think they make a ruler/cutting combo that allows you to lock the ruler in place for cutting unusual angles. I can't remember who makes it or if it would work for you though.