However, you might find it difficult to shoot handheld (at least at iso 100) if your maximum desired aperture is f/16. None the less, I'd agree and even consider the 90mm Optar and just use faster film.
This is reminiscent of the Gowland Aerial 4x5, which he sold new for about $600 (many years ago).
Now we need an inexpensive but relatively accurate optical finder. I'm thinking something like the simple, plastic Russian 85mm finder - but sized for 90mm on 4x5. Maybe a 35mm viewfinder (for 35mm camera) would work well enough; you'd have less than 100% on the vertical axis but that might make up for any lack of alignment between lens and viewfinder. Time to dig through my "stuff" box.
If they're stalled on reaching their goal, they should add "lomagraphy" and "iOS compatible" as keywords.