What about a thick teflon blade bolted to a metal core? Teflon is kind of expensive, but if you could attach a thick sheet and then grind/cut/polish to your needs it might work.
Could you prototype with something cheap like HDPE? Or maybe Delrin?
You can get sheets of each of the above at http://www.usplastic.com/catalog/pro...uct%5Fid=10533
HDPE and Delrin and a bunch of other materials through the same place. You can get Titanium from http://www.smallparts.com
The idea would be to cover the face of the device with the sheet of teflon and leave say 1/2 inch on the trailing side so that you have less of a chance of getting the solution up on the metal parts. You might still be able to use Stainless for the metal, it just wouldn't need to be as exacting. If you used stainless for the body, you might be able to reduce the amount of teflon needed. That's all going to be dependant on the stiffness that you need.
In theory you could just use a straight bar of metal and let the plastic do all the work. The advantage would be that you could replace the blade if it was damaged.
All the same might go for a stainless body with Titanium scraper. Once again this would off a replacable blade for a lesser price than the whole unit.
I took a quick look around the web to see if I could uderstand the needs a little better, but there are few pictures of what you are making, so I only have a rough idea. Essentially it looks like this device is a scraper that is positioned at an exact level above the paper. A large amount of the solution is applied at the edge of the scraper, and the paper is pulled through to apply the coating evenly over the top of the paper. Speed of pull, thickness of solution, and scraper height above paper determine the thickness of the coat (other factors may apply). The coating needs to be uniform within about 1/1000 inch to make sure that you get even tones when it is exposed to light. A thicker spot will give a higher concentration of silver, which will give a different tone than a thinner spot when the same amount of light hits them.
Is my theory correct? (in it's somewhat simplistic way)
If it is, then you may be able to makes the base metal structure in a modular way so that many different widths can be accomodated simply by switching the length of the plastic scraper (and careful setup of the scraper). Since you are using plastic for the scraper, you could put in adjustment screws in order to fine tune the edge all the way across the device. You could probably provide the same adjustments to the metal blade, but it would take more force to make the corrections.