Why not try sourcing drafting vellum to see if it can be coated. It was expressly made to be quite translucent, to allow the prompt exposure when making blueprints.
I am pretty sure that modern blue prints are not far off cyanotype, although the last ammonia based amchine in our office yielded a direct possitive image.
I was cyanotype printing last night.
I was read a reference in Keepers of the Light while waiting on the print in the uv box, and channel surfing on the tv. Isn't cyanotype printing great that way?
Crawford mentions that Herschel found coating solely with ferric ammonium citrate and exposing and then adding the ferricyanide after exposure yielded a positive.
I have yet to try that one for myself.
It does sound intriguing.
If succesful for me it could lead to one off cyanotypes in the spirit of what a degaurrotype delivers, direct from the camera.