I personally have printed on the Epson 3880, and have a Epson 1400 for my home and personal use. I would recommend ProPhoto RGB, and doing the 16bit. The ProPhoto RGB does better with color, so this may not really affect the BW images you have. Just to be safe I would use the ProPhoto RGB. The printer can handle it, and I have had no problems with it thus far. FYI: The only problem I frequently have is that the blacks print extremely dark on the Epson (its known to be a problem), so I am always altering the blacks in test prints. You may have already experienced this.
This link may better help explain the differences of Adobe RGB vs. ProPhoto RGB :
For the Epson 3800/3880, you might gain some insight from Eric Chan's print workflow guide. In it, he goes over several different black-and-white workflows and discusses color spaces.
I believe he wrote the guide before he joined Adobe, but Eric is now one of the key engineers behind Lightroom and Camera Raw.
I'm probably on the far end of your opinion spectrum but I just completed a big print job (2 ea. of over 1600 9x9 b/w aerial scans) using our fairly new Epson 4900. Before I started I called Epson and asked them how to approach printing grayscale with their machine. All my other work (mainly color to a HP Z6100) has been in SRGB 8bit and their response was - convert to that. So I did, and the results were outstanding. But, I also had an overall darker print than I had onscreen by almost 20% +/-, using a Spyder 3 Elite calibrated system.
So yeah, I guess you could worry about these monsterous color spaces and bit depths, there is beauty is keeping it simple...
Thanks. I glanced at Eric's web site, and saw where he says: "I prefer to edit images using the ProPhoto RGB working space. Adobe RGB is also a good choice" which I already knew.
He doesn't really make many recommendations for BW printing, at least regarding the color space, except to say when using the Epson printers' Advanced Black and White driver (ABW) you should set the Printer Profile menu (not the image color space) to sRGB. A bit counter-intuative, but he explains why.
I glanced at (and will read more of) the Luminous Landscape article. Again, I generally understand the ProPhoto RGB advantage for color images, but the article doesn't address black and white.
ProPhoto RGB could be a bit better in rendering BW images, but my guess is it's probably like discussions about Dmax and sharpness: they might mean something if you plan on looking at your images through a 20x loupe or an electron microscope, but may not really be that important to creating images with presence.