This would, I agree, be a tiny niche, but it's something that interests me personally.
Hi Roger, no, I did not misunderstand you. Like you, I am also intrigued by the possibilities of direct positive paper, and the prospect of being able to print directly from slides, be they monochrome or not, is something I hope we will still have some time in the future. The orthochromatic nature of the Harman DP paper is slightly problematic for printing from colour slides, since essentially reds will turn into blacks and skies will burn out. One could temper this with some colour filtration, but not to the extent necessary to achieve an even panchromatic response with reasonable exposure times. But this is off-topic.
I agree that B/W slides have a place in terms of projection, and as part of a digital workflow. But we started out on the subject of XP2 Super, which apparently is blue and not grey as a positive developed in E6. That impacts the projection use (for better or worse). As far as I'm concerned, whether I start with a blue positive or grey negative and scan into a digital workflow would make little difference, but for printing directly, it is worth considering that paper is most sensitive to blue, and this may cause contrast issues if your film is a blue-based image. In that case it could make more sense to use a silver-based film rather than a chromogenic one. I hope someone has enough Harman DP stashed somewhere to try it for us.
The other possibility is replacing the CD-3 in E6 with CD-4, but following the rest of the E6 pathway to achieve a reversal. In theory, shouldn't that allow the correct dyes to form, that will be black rather than dark blue? That should then give you a straight grey positive instead of a blue one.