I believe there are 3M cartridges that are rated for isocyanates, but they require changing/replacing every hour of spraying, *and* they require a properly vented work space.
I would definitely be using a full-face respirator at the very least, rather than a half-mask style.
That being said, a supplied-air system sounds way safer.
I get very nervous reading these threads, about people using proper safety procedures - especially when Spraymax seems to be a popular choice due to being much cheaper than the investment required by an HVLP setup.
Don't cut corners, people - if you don't have the money or space to do it safely, don't spray clear yourself!