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...

Thanks again.