I've been following this at a distance, since I do not make digital negatives. However, FWIW, here are a few comments on multiple inks running the full length of the scale. Note first that early on with the Epson R800/1800 I designed the "3-MK" setup that used 3 MK inks to print B&W positives. MIS marketed this approach. Subsequently, and in more recent times I've used a single MK in the 1400 with the Eboni-6 variants to hold down the warmth of dilute carbon inks.
My first observation is that, while I used 3 MK inks in the R1800 setup, it was to hide the microbanding. In terms of roughness or graininess, a single MK is smoother than multiple MK inks firing at the same time. To keep the highlights in positive printing smoother, I always start with only a single MK and then blend in the others as needed to hide microbanding. It appears that an ordered, non-random dither pattern is or can be made to look smoother than an actual random pattern, which (according to Roy) is what one ends up with once multiple channels are firing at the same time.
One of the problems I see with inkjet digital negatives is that the variable dot systems use the small dots for the positive highlights and large dots for the black end of the scale. When a negative is printed, this becomes reversed and backwards. It may be, however, that at the 2880 setting in QTR the smallest dots are used throughout. Roy would be the best source of information on this.
It would certainly be a benefit to keep the printer at the 1.5 pl drop size. I do think one might be able to see the transition to the larger dots, particularly if all channels are the same so that transition occurs at the same point. It may be that, even if the 2880 setting does not turn off the variable dot, holding the ink load down keeps all channels at smaller dot sizes. I just don't know what those internal algorithms look like.
Holding down the ink volume on glossy paper is useful. The reason for the early MIS UT1 inkset transition to UT2 was simply that pigments had become glossy compatible and the one cold toner was flooding the paper, causing roughness. The same thing happens to uncoated watercolor papers -- it's easy to overload them. One of the jobs in profiling and setting ink limits is to look for signs of paper overload.
Again, while I do not know the actual algorithms, it has been my assumption that darker inks in the highlights limit the ability to print the full scale of the file smoothly. That is, the individual dots only have the ability to vary in size up to about 5 different sizes. So our prints, particularly those made of one ink, are mostly half-tones. They make more shades of gray only by using a larger area and putting more of less dots into that area. The darker the inks, the larger that area must be to print visually different shades of gray. It may well be that that dither pattern is more the source of roughness than the dots themselves. If all the inks are in identical curves, each channel may be using the same dither pattern, accentuating whatever pattern is there.
This seems to run counter to the earlier observation that more dark inks firing at the same time turns the ordered pattern into a random one that is seen are rougher. Some empirical experimentation may be needed to see which way this cuts and at what levels.
With the original 3-MK setup, I manually drew the curves and made them not totally congruent so that the dither patterns would not be aligned. My concern was actually more that I wanted to avoid what would might be a moiré pattern both between the individual dither patterns as well as between the combined pattern and image file content. So, the curves ended up weaving around each other to a limited degree.
I totally understand the desire to make good digital negatives at home with an inkjet printer. For my 16x20 silverprint work, however, I ended up not using inkjets. This was some time ago, but I found the Fujix Pictograph (sp?) machines that Samy's and other outlets had made excellent and cheap 8x10 inch digital internegatives that I could enlarge to 16x20. It was cheap and easy to modify a 4x5 Beseler to take 8x10. There is an Apo-Rodagon copy lens that is a fraction of the price of their enlarger lenses that is still diffraction limited at 1:2.
At any rate, as I said, FWIW, these are some possibly relevant observations.
Good luck with the interneg project.
Paul