I don't know about the 188 version, but maybe the 308 (which I've been working with) is similar enough.
Through my testing, I have found that Hahnemuehle's recommendation of Velvet Fine Art as the Media Type to be correct, for both printing through the standard RGB color driver and the ABW driver. For ABW, I recommend using Neutral or Light for the Tone menu (instead of my usual recommendation for Dark). Otherwise, the shadows will be overly compressed.
For both RGB and ABW printing, I leave the Color Density as its default value of 0.
I confess I wasn't overly impressed with HPR308's depth in my early 3800 tests. I am seeing a d-max of 1.64 in both RGB and ABW printing (L* = 17). I expected higher. I am used to seeing ABW prints with deeper blacks than corresponding RGB prints, so the mere fact that d-max is the same for both is a little unusual to me.
Perhaps other regular users of HPR (either 188 or 308) with K3 inks can comment on whether they're seeing anything unusual.