Thanks for the input! 
I’m currently trying to take a more complete measurement of the film response curve, using the 5 target shots from the post above, taken at increasing exposure levels from -1EV to +3EV.
I wrote a small script that extracts the average channel value from each of the 6 gray patches on each of the pictures.
Then i did the same sampling from a digital picture of the same target, shot immediately before the negative was shot. From the 6 patch values of that single digital pictures, i derived the corresponding values of whole range of exposures -1 … +3 by simply halving or doubling the initial values.
Finally, i made a log-log XY plot with the digital values on the X axis, and the corresponding negative values on the Y axis, here’s what i got:
which is surprisingly close to what theory would suggest
(note that the negative pictures were white-balanced on the film border, hence channels coincide at their highest value).
Unfortunately, this range of exposures is too small to show the real limits of the film, but it seems that there is already some slight “bending” at the extremes. To be able to “zoom out” on these graph, and see what happens on more extreme values, i need to shoot a gradient with a broader dynamic range.
For this purpose, i ordered a small 8x8 LED matrix, that i plan to drive via an arduino, starting from the array fully lit, and turning off one pixel at a time, at increasing delays. All this while taking a 1sec or longer exposure, shot directly at the array. This should give a 64-steps gradient spanning a huge range of light values.
I expect to reach the limit of the lens (due to lens glare) much sooner than the limit of the film; in this case, i’ll split the work in two, and take separate shots for the upper and lower limits.
I hope to observe that all the channels behave the same way at the extremes; this would mean that we won’t need three separate adjustments for toe and shoulder. Also, we’d be very lucky if the curve behaves simmetrically on both ends … we’ll see 
Maybe, better handling of over- and underexposure, could also lead to better and easier-to-achieve color accuracy, because sometimes while adjusting our negative conversions we might be misled by some color cast in a highlight area, and to fix that we throw the rest of the picture off the rails… 