"I can't really think of a way to vary the duty cycle using this arrangement. "

If the LEDs are all connected to Vcc, and the other side to driver.
Insert a MOSFET between Vcc and the LEDs, drive it with a variable width pulse from the '51

type 2:
If the LEDs are all connected to ground, and the other side to driver.

Insert a MOSFET between Gnd and the LEDs, drive it with a variable width pulse from the '51

Erik