dhenry:
2) reroute your pwm signal to the hs595's OE pin(s).
Fantastic! Never occurred to me that you could just PWM the whole chip using its output enable pin... So I never needed those transistors at all... man, I've been doing that PWM transistor trick on everything, completely redundantly. D'oh! 
Grumpy_Mike:
(snip) ... a filament can not react fast enough to be dimmed by PWM due to the thermal lag. The control region is very narrow.
I'm not sure I understand you, Mike - haven't domestic dimmer switches been using a form of PWM to dim our incandescent-laden chandeliers for years? The thermal lag just acts like a low pass filter, surely. Or do you mean that the light response may not be linear, or may be inconsistent eg. may exhibit hysteresis when ramping up and down?