I don’t have an answer to your question gerardk, other than to say that the spectral response of film is completely different to digital. The response of film is non-linear and also varies for each of the dye’s. The response of a digital sensor, on the other hand, is totally linear. Most digital camera’s are overly sensitive to red anyway and I suspect this may be the reason for the often ugly results you get when using a gold filter.
I’ve simply stated my experience with gold reflectors and have noted that others have commented similarily elsewhere in the past. I have no hard and fast reasons why this should be so and readily admit that I may even be doing something wrong. I’d be delighted if you or others would try it out and see what your results are.