Xilvar, mapping from one space or profile to another using BPC merely matches the K point of one to the other in the manner you mention. What a profile, or a conversion using any intent or method, can not do is open and linearize portions of the scale that are slammed together by the nature of the device, ink, and paper combo. BPC is working with the math, and can't know that those lower L points it's working with can't be pulled apart by the device.
My suspicion is that if you were able to take a look at the profile tables indicating the lowest measurements of the device behavior, you'd find they are all blocked up. In order for a profile to correct luminosity there has to be some delineation in there in the first place for it to work with, then it can do it's best, and the various intents and BPC use would make their benefits apparent.
That's my best guess...
But of great interest in your post is the obvious superiority of using Quadtone RIP for B&W prints from UC inks in terms of photographic linearity. There are other benifits as well which are off topic, but people should be encouraged to try it out. For only $50 and the potential for greater capacity from UC inks for B&W, it really is a great development for B&W artists.
My best suggestion for Epson driver B&W work would be to go back and make tests using no profile to find driver and media settings that provides separation at every step down to 100% on your particular paper, then profile over that. Profiles are best at remapping over somewhat rational channel behavior, for lack of a better description...
Tyler