I don't have any 20MHz xtals to try.

FWIW, I was able to get an 18F26K80 running at 80MHz with an external 20MHz osc into OSC1/CLKIN (using CONFIG = EC3).
The 4x PLL worked using either CONFIG PLLCFG = ON or CONFIG PLLCFG = OFF and setting OSCTUNE.PLLEN = 1.

I didn't even have a cap on the VDDCORE pin, and it worked with VDD down to 3V.

According to the datasheet EC and HS modes use the same clock tree, so I would think if you're using HS2 mode with a 20MHz xtal it would work too.