It turns out I'm not alone in lacking a term for this tradition:
These photographic ‘foregrounds’ are known by many names. Recently Michael Quinion of World Wide Words,  noted quite a few awkward but descriptive phrases: ‘end-of-the-pier painted boards into which you stick your head to get photographed’, ‘head through the hole’, ‘things you stick your head in’, faces in holes, face cut-outs, ‘head through the hole photo booths’, photo cutout boards, comic foreground, carnival cutouts, lookie-loo, mug boards, faceless cutouts -- and even had a new suggestion from a reader – ‘Headleys’ for the surname of the person who first asked Michael about this topic!
Vivian Marr of Chambers Dictionaries gave Michael the French name – “‘passe-têtes’, essentially places to put one’s head through” which is the one I’ve adopted now. Very clear, I think and quite Canadian sounding, but I’ve seen other terms on-line now too – arcade photograph and ‘people posing in wood cut out bodies’.
There is some question about who ‘invented’ these ‘head in the hole’ photographic props, but it seems accepted that Cassius Marcellus Coolidge (1844-1934) popularized them, if he didn’t think them up all by himself.  (He’s the fellow who painted those ‘dogs playing poker’.) I’d be interested to hear of any contemporary references to his prop work or to his company.Now my task is to find some good examples, armed with a search term, especially for amusing foreground with a Mardi Gras theme. Starting with "comic foregrounds" and following up with "head in a hole," I find: