In the Postcard Exchange - Absolutely. Cyanotypes use UV light, which is quite "optical".
Two types are allowed:
1) lumen print type cyanotypes. This is the exception to requiring that the image originate on film, although you could consider the cyanotype itself to be the "film"; and
2) images that started out on film, and are printed from there.
Most would choose to directly print a film negative by making a contact cyanotype from it, but if you wanted to scan the film negative and then make a digital negative from the file, there is nothing wrong with that. That would make working from a 35mm negative - or for that matter, an 8"x10" negative - practical.
I'll leave to you why you would want to do that for postcards!