I'm posting the resolution to this for the benefit of anyone who might come across this problem.

After trying several alternate fixes, what finally made it work was installing a 103 resistor across OSC1 and OSC2, along with the normal resonator circuit.

I did read the 18F252 manual long before my original post, which is why I bought the chip in the first place. On page 19 of 316, I see there is no internal oscillator to switch FROM, and HS IS the correct setting for external 20MHz resonators. The device does not seem to have 20MHz resonators listed, but FYI, mine now works! Also, code assembled and loaded without setting config bits within it. Manually setting config in the programmer worked, which I verified by reading the config back out of the programmed chip. MPASM was not a problem other than the length of the file path and it appears to have made my code about 5k longer, cancelling much of the gains of moving to a bigger chip. My blinker test code worked and everything else went as normal. I'm off and running.

I posted this hoping to avoid hammering out the solution to yet another PIC mystery and save some time.

Other than insulting my intelligence (READ! Why didn't I think of THAT?), giving wrong information, and not addressing a single issue in my post, this has been a great help.