Take a look at Wight's driver design based off of the 7136 chip. It isn't 100% true constant current, but very close to it. The PWM is virtually undetectable, although it is still there. I would consider this driver to be a good starting point for what you want to do, unless your goal is 100% PWM free.
I would take a read of that thread.
I believe this to be the most economical solution, and easiest to adapt to new driver designs using existing open source attiny software. JonnyC has begun his FW port to the attiny25 (I've tested semi-functional versions), and in turn the 45/85 as well, so if you want more FW space I don't think that will be a problem. Don't bug him too much about it though, as he's busy with his real job right now as well. Just know he's working on it.