Well I've just printed from Lightroom with the Greyscale box checked and colour Managed by Printer and I immediately have prints which have a brown (magenta?) cast!
I then tried with the same settings but using the ICC profile for the paper (Harman Luster) with the same media selected ie PhotoPaper Pro 2 which if I remember correctly is what Harman recommended, and got an identical result.
Finally I tried changing the media setting to Luster Pro and got a slightly pinkier shade of brown.
Finally I removed all the colour inks from the printer and tried to print again with just the black ink but the printer wouldn't allow me to do it!
I'm guessing that even on the greyscale setting the printer is using other colours?