Interesting, I never knew this existed, the red film thingy that is.
Shooting through the back door has never occurred to me, apart from the colour cast there could be a slight advantage with portraiture, any wrinkles will be smoothed out by the slight out of focus happening, I would assume. Skin tones would be a blast though
As for the E6 film running through a C41 bath, I'm not too sure some places would like that happening on the sly.
I worked with the bath controller on a large C41 dip-n-dunk machine. Sometimes we would have some serious amount of E6 going through the C41, I'm talking 4x5" and 8x10" sheets in numbers.
Always after these had been processed, the bath had to be stabilised again. Meaning we couldn't use the bath for serious C41 processing for about 1½ hours minimum. Only after a first control strip had gone through, been read, then the bath had some adjustments made, then another control strip through to check if the corrections brought the developer bath back to standard, could we put more film through.
This absolute correct bath was required as we used a lot of C41 Kodak Color Print film, to produce colour corrected transparencies for graphic reproduction.
So I know that cross processing will alter a bath somewhat, probably not too much with the odd roll, but if someone came in with 10 rolls in a small mini-lab the situation, it could be interesting.