As I think about it more, I realize I couldn't really use the µC to monitor the µC—how is it supposed to know what the "correct" frequency is when its internal osc is is the one that's off? Silly me.
I have a frequency meter—the MS8268. It reads 59.98 Hz from the wall, but gets nothing from the CLKO pin (which I know is putting the clock out, 'cause an LED glows when attached). I should have one test lead on CLKO and one on ground, right? Maybe it's just cheap Chinese junk...
I think I'll get the TCXO from DigiKey. All my code needs is an accurate 1Hz (or 0.2Hz, even) interrupt to count the seconds. I don't think an external RTC would work, as I want to be able to set the clock if needed.