Thanks everyone for the responses. I used an approach that combines jovo's and DWThomas' methods -- enlarger at maximum height, stopped all the way down. I placed five pennies over a test strip and exposed in one second intervals.
I didn't have a translucent white filter, but found some clear translucent material and cut it to size. A one second flash didn't register by itself, but it did bring out shadow detail I hadn't been seeing when I made a test image.
If I find myself doing this more than occasionally, I'll probably look at Joe's method (always good to hear from Joe) or the gizmo that pentaxuser mentioned...
You can also pre-flash in camera with a bit of white plex/whatever in front of the lens.
Meter through the 'whatever' and give a white-light exposure xx stops less than metered. This works with pinhole or lensed cameras.
Flashing for paper negatives is very different than flashing for paper prints. For a print you want to flash to a standard "imperceptible almost-but-not-quite-pure-white." If this is what you want to do because of hidebound tradition then give 2 1/2 stops less than metered.
However, when pre-flashing paper negatives it is better to flash to a density of 0.2 OD to get you well off the toe and on to the beginning of the straight(ish) bit of the HD curve. For an 0.2 OD flash give an exposure of 1 2/3 stops less. [Obviously not what you would normally want when making paper prints, but not a bad idea if you are making enlarged display transparencies from negatives where shadow detail is no problem but holding highlight detail can be the devil.]
The above exposure figures are for Ilford MGIV RC. But they hold well enough for other Ilford VC papers. They also assume you have calibrated your system so that a metered exposure of a white wall will result in an 18%/ZV tone.
You can also post-flash - the order of the two exposures is rather immaterial.
I have a couple dozen sheets of very old (bought in 2004) Ilford Multigrade IV RC Deluxe (MGD.1M Glossy) paper. It's 5x7, and I'm going to expose it in a 5x7 pinhole camera. I'll develop a part of an unexposed sheet tomorrow to see how badly fogged it is. If it is fogged, will it still need to be pre-flashed?
Also, how should this paper be rated?