Update:
Per the advice of a smog tech, I replaced the coolant temp sensor and patched a tiny vacuum leak @ the throttle body. Got the cat really really hot and had it tested again. Passes no problem @ 15mph. At 25mph NOX is below average, CO is below average, but HC barely goes over MAX (MAX is 74 and I got a 79). With an almost empty tank of gas, I dumped in two quarts of denatured alcohol and had it tested again. Results were AWFUL!!! The HCs jumped up to 250, NOX to 600 (from 150), and CO went up to .70 (from .10). The smog guy called me stupid for even trying the denatured alcohol trick. He advised me filling up with 87, dumping in a bottle of Amsoil Fuel Performance Improver, driving it to empty, then filling it back up with 87 and getting it tested again.
This is driving me bonkers...