So back to C-41 neg's for a moment... a while back I calculated the average density of a well exposed C-41 negative, ≈5.5.
My thinking is to measure the incident light at my baseboard, and contact print a 4x5" C-neg onto b&w pan film, applying this density as a kind of filter factor. From the incident reading you use f/1 for your exposure time (since there is no aperture to attenuate the light), apply filter factors for the Wratten filters being used and also reciprocity-failure corrections. Ideally the exposure times should all be the same, so perhaps apply ND or if you're using an enlarger as the light source, adjust the aperture.
This should get in the ballpark of properly exposing a C-41 separation, and if I take 2 bracketed shots of each separation to give me 6 total sheets I can develop them all at once in my MOD pro-plates 4x5" processor.
Now.. am I missing something? Sure a C-41 negative 'looks' different, but what is so fundamentally difficult about making a positive separation from it? Does the mask skew the balance of light so that each layer requires a radically different exposure and if so, wouldn't that be fairly constant, thus allowing you to apply whatever skew there is across the board?
If you've got a set of good, straight-line, separation positives, these can be projected onto X-ray film to make the enlarged negatives. At least that's the plan...
One last thing of tangential interest that I'll add... the old Pinatype process required separation positives to make dye-imbibition prints. This is because it used a planographic matrix as opposed to a relief matrix. This is exactly the kind of a matrix used in bromoil/oil printing, wherein there is no hot-water etch and you're left with an image of untanned & tanned gelatin. Since this was under a positive, the highlights are tanned and thus do not imbibe liquid so easily, whereas the "soft" gelatin in the shadows does. The obvious discrepancy here is that the tanned gelatin of a relief matrix clearly accepts dye, so how are we supposed to get clean highlights? The key is in the dye itself. The Pinatype process used a different class of dyes, proprietarily known as "Pina dyes"; the chemical structure having never been disclosed. However, dyes with this property are known and in Friedman's book. Figuring out a planographic-matrix imbibition scheme would allow you to make prints from positives.
But I include this mainly for historial interest, unless of course someone wants to take it upon themselves!