A little research at work later (working with people of varied ages from varied locales comes in handy--I don't know why I hadn't used the co-op for ersatz linguistics research before:

A brief recap from memory:

Most of the people my own age (up to about 28 years old) had never heard of the term "make-work" before, and this included people born and raised in Columbia, S.C., southern New Jersey, Memphis, Tennesee, rural Iowa, and Portland, Oregon.

However, everyone older, including my 29 year-old friend from the Bay Area, had heard of it--one guy from Chicago, a woman originally from coal country in PA, and a woman originally from Albany, in my limited sample. All identified it as something that members of their community had been known to use (the woman from Albany excluded, as she wasn't sure where she'd heard it), but in all cases the word was either singled out as rare or a feature of the speech of the elderly. So, in my limited research, it does not appear to be a regionalism so much as a word that's a bit on the archaic side, or rapidly becoming so.

Further input is requested and welcomed.

Also, the coal-country woman (with whom I empathize, my Slovak forebears having more or less wandered from Ellis Island to Nesquehoning, PA around the first world war) also provided me the link to this site with a link to the "Coalspeak dictionary," an entry of which is provided as a sample:

"mango : green bell pepper. To the rest of the world, a mango is a tropical fruit. In the Coal Region, a mango is a green pepper. I don't know why this is."

The Devil's Dictionary it's not, but it's not without its charm.