PE, did you consider the use of sodium benzoate? It should not interfere chemically (redox-wise), it is nicely soluble in water etc.
I have another question, a bit off-topic, but anyway-if I understand well, the incorporation of dichromate directly in the glop before coating (instead of putting the tissue to "swim" in it, thus handling loads of dichromate) is undesirable because there is the "dark" reaction (reduction) occurring in the coated tissue. BUT, there is sucrose as a plasticizer and thymol, both having some "nice" reducing properties, giving us the reduced Cr(III). IF we replace sucrose with sorbitol, thymol with, say benzoate (or something similar), the tissue with dichromate should be stable for a long time, provided it is kept away from UV light. Correct?
Is there anybody who tried this before (I am not into carbon, although intend to, someday...)?