What are they like with a load of some sort (a few hundred ohms maybe)?

Yup, gotta test them under load, usually at least 10% of rated output watts/amps.

Two things I might try to run them and not tie up your variac... put them in series with 100W lightbulbs... or stack 110-220 and 220-110 adapters and see if the losses drop it to 100. Might work even gooderer if you find a 115-220 and put a 230-110 on it... if they're slightly off 2:1 ratios and not just 2:1 and telling you what you wanna hear.

Well... one of the adapters that was sent is a 100-230v wall wart and it puts out almost exactly the rated 12v. I also tested one of my 120v Roland 9v adapters and not under load it was putting out a bit over 12v.

I don't mind using the variac for now as this is the first thing I have actually used it for. The company I used to work was giving away a bunch of stuff when they consolidated with another location and it is one of the many things I grabbed.

I'll see if I can do some under load tests.

Are they switching or transformer PSU-s? And usually deviations in mains voltages are allowed. Today 2,5-5% in developing countries even 10%. but with old hardware you better not risk it.

FWIW, I've been pumping 120v through the veins of my 100v N64 for well over a decade without issue. I'm not saying that all 100v devices will be as tolerant, but some (possibly many?) seem fine at the slightly higher voltage.