The quads cross two joints--the knee, and the hip. If you're not 'allowed' to move the quads against resistance through the ROM at the knee, you will have to do it at the hip via what might be called 'un-hinge movements.' You could do lying leg raises with resistance (in the form of ankle weights, bands, KBs hooked over your feet, etc). You could do the same movement against cable resistance (via an ankle strap) while standing.
Will these movements blow your quads up like a heavy set of squats? I highly doubt it. But with proper attention to MMC, you can probably use them to maintain quad size until your patella tendon heals.