The driver didn't make it out on 4.4Lite, so it might be risky to use it.
(Although I suspect you are correct w.r.t. no AT&T/USL code. I think the
 problem was that all the BSDish serial drivers in some sense decended from
 the old dz.c on 32V.)

If it's any consolation, I wrote a driver for these guys way back when
(it would have AT&T derived code in it) and it was pretty straightforward.
I used dma for all output, since then the dhv and dhu look the same.

Good luck with it, rick