Instead of driving all the LED's off the PWM pins can you just PWM one pin and put a transistor on it and use it for a common ground for all the LEDs?
Yes, but draw out the circuit so we can tell if you are still handling individual LED current limits correctly.
I did this with the large LED blinder I made a few months back. I used 3 transistors from 3 PWM pins so that I could control the overall color and then used the extra digital pins to control whether each LED was on or off. Theres a video on my youtube account if your interested