what about looking out for a toyo vx125? You get the stability of a monorail(albeit a very light one), a collapsing rail for portability, and geared movements?
A friend of a friend has one, I've only seen it, never used it, but gosh, did it look sweet.
they come up from time to time on the used market, generally ebay, but they get snapped up pretty quick, and for a pretty(often shiny too) penny(or two )
just an idea
p.s. there's also arca-swiss, quite a few people like them cause you can collapse the two standards down onto the small rail section, and just screw in another longer section if you needed one.
I know neither of these are folding cameras, but the 90XL lens isn't small(I'm assuming you have one). If it were my choice, I'd be on the lookout for a used VX125(or the vx125b, non-geared movements/same size)