Thanks to all for the many valuable replies! (My apologies for posting this, then taking so long to get back to it.... just got very busy with non-photography stuff, is all.)

For this work I mostly use two 35mm systems: 1) vintage Spotmatic gear and 2) Exakta gear that's even more vintage (vintag-er??). The Exakta has a lovely magnifying finder called a Magnear that provides very high eye relief but works in waist-level position. Any inverted post arrangement has to keep the camera upright to use this finder -- on the first page someone did post a pic of an inverted post set-up with the camera upright.

Thanks again for the replies. You guys and gals are amazing, and have given me a lot of options to think about!