I've not done this myself, just thinking aloud, so to speak.
Could you cut your fabric into 7 1/2 squares as kassaundra has said and fold them I to quarters. Instead of the omni ruler, which thuimblebug6000 said can be damaged, do it the old way and use a ceramic bowl or plate.
Placed against the quarter folds the wavy or pinking blade could be run straight around the curved edge of the plate not damaging it, keeping your fingers safe and not having to do the job twice; once by rotary and then scissors.
It may or may not work, but I can't think of another way besides what has already been said. Best of luck.