It may be possible but probably you have to write the software yourself, completely. The only library I know of (
http://code.google.com/p/tlc5940arduino/) uses a rather complex setup of timers and PWM channels to drive the chip's pins. It's written for the UNO so with the additional capabilities of the ATmega2560 MCU you should have enough hardware resources (if you don't insist on having all 12 instances completely independent, you don't have enough timers for that) to do it but no software support yet (again: as far as I know).