On a lark on day I typed 5x7 plate holder into google and a listing on craigslist appeared ...
9 DDS holders with 4x5 wooden plate holder inserts, 5x7 sheet film inserts and one for 2.2 x 3.5" plates arrived today. Once I get my camera back I'll post some shots of the innards just in case anyone else needs a visual reference. They're slightly thicker than the wooden sheet film holders stamped Folmer Graflex Corp and 5x7 Eastman Film. Otherwise they are the same dimensions as the more modern holders.
It doesn't look like the 4x5 inserts need much modification for wet plate either.
(soooo relieved this part of my search is over, what a PITA it has been)