Regarding Eboni-6 and diluting HP Z3200 PK, I use this combination in an Epson 7800 and have used them in other printers for quite a few years now. For the 7800 setup, see
http://www.paulroark.com/BW-Info/7800-EbHP-2013.pdf Eb6 has not been formally tested in its full dilute form, but with carbon pigments in general, dilution does not impact its lightfastness. I did analyze the
http://www.aardenburg-imaging.com/ test of Jon's carbon set with this in mind and posted the results in the appendix (last page) of
http://www.paulroark.com/BW-Info/Eboni-6.pdf . (Frankly, the concern about dilute inks doing worse on fade tests seems to come from the old style dyes. Even Claria light inks hold up essentially as well as the darker versions.) Carbon is such a solid pigment for high end B&W, it seems odd to me that more serious B&W photographers are not using it. It's their loss. When you also consider how well Eboni-6 can print on Arches (uncoated) Hot Press (or Cold Press) watercolor paper, thus avoiding the coating weaknesses, the inkset is rather compelling.
As to the HP PK, I use the generic dilution base version C6b for diluting it for Epson printers. MIS sells this pre-mixed at
http://www.inksupply.com/roarkslab.cfm .
(Don't use version C6a, it was for the 7500 only. BTW, I make no money on all of these mixes that MIS commercializes. I have used these bases for years with never any negative issues. The formulas are all published and open. The one warning I'd make for those mixing from scratch is to use fresh distilled water and 99.7% glycerol.)
Back to the HP PK, note that the HP pigment inks print well in all the Epson printers I've tested them in. (That does not include the 3880.) The OEM HP Gray and light gray are a bit cooler than the PK-diluted versions.
I would not mix the Eb6 and HP pigments in a single ink. Different electrostatic coatings on different pigment types can accelerate separation. I avoid any mixing of different pigments within a single ink when the target is wide format. The inks mixing on the parking pad is not a problem. They are compatible, but presumably have different electrostatic signatures in the suspension.
(I have not tested an Eb6 - HP blend in a single ink. My experience/conclusions come from years of mixing and testing MIS and other companies' carbon + color blends for neutral B&W desktop printer inksets, where the movement of the ink carts with the head assembly avoids the separation issues with continuous agitation. Note that my "generic" base formula is very specifically made to avoid incompatibilities, and it does so very well. Note also that in a centrifuge test of settlement rate, the HP dilution with the generic base settles more slowly than the OEM HP Gray -- no magic, just slightly higher viscosity.)
Hope this is useful.
Paul
www.PaulRoark.com http://www.paulroark.com/BW-Info/