At 10MHz, I get the following:

Open: R 255 S 003 Z 248

Shorted: R 232 S 255 Z 001

Decent 50 ohm termination: R 001 S 127/128 Z 121

At 3 MHz my 100pF +/- 10% capacitor reads164-j530 ohms on the Impedance
display ... it seems to get X right... Furthermore, setting the meter
to capacitance makes it read about 108pF... but there's this big
resistive component.

In the test menu, this cap reads R 255 S 023 Z 247 at 3 MHz

So maybe I just need an alignment... I suppose it wouldn't hurt to try
it if I can scare up some precision terminations.

I wouldn't consider these numbers "way off" ... this reading of
resistance at low frequencies had got me worried that something was way
wrong, but will an alignment fix this? I guess I can check through
the W8JI alignment procedure with the proper loads before I start
twiddling things.

Dan