Hmm, really interesting now.
I myself haven't noticed much, if any, image degradation in the past when placing colour correction gels behind the rear element using Blu-Tack that is.
But, we were more interested in getting transparencies to dead correct colour for four colour printing. Exposure and possible image degradation, would have just been adjusted out a bit further down the track, as by the mid eighties, all trannies were used for colour stats for pagination to get page by page approval from the art director (sigh), then they were drum scanned for electronic pagination.
Although I wonder about image degradation happening enough with the lens being used in its complete form, as opposed to the front half of elements missing.
This has given me an idea for portraiture with my Fujinon f/5.6 250mm W (wide) lens on 4x5".
Bellows length could be an issue with my Shen Hao, but shouldn't be a problem with the Toyo 45G monorail.
I feel a bit of fun experimentation coming up.
Mick.