I'd always blithely assumed that "Dou way" was, basically, "thou away" as in "go thy ways" = get thee gone, thus that the 'd' of 'Dou' was actually (or was derived from) an edh. Now I think about it, though, I suspect I may be completely wrong! Unfortunately, my Middle English dictionary is of no particular help here - it has just sent me round in several circles of cross-references - whilst Chambers and the OED give suggestions for 'dou' and/or 'dow' that would be more likely to encourage the gentleman than otherwise. So no joy there either. Rats.

Isn't it a lovely motet, though? You probably know this already, Leeneia, but it apparently survives on a half-burnt sheet of parchment (the Cotton Fragment). The British Museum entry is:

It dates from the 14th century and was apparently a flyleaf in Cotton ms Titus A XVIII. It is dated 1349 in a document at the bottom of the recto side and is probably from Adel (Addle in Yorkshire). The notation is English mensural notation on a five-line red stave. The work on the recto appears to be a motet. There are no minims or Ars Nova traits in the notation, which suggests a date c. 1300. The script is a typical minuscule such as we find so often in musical manuscripts of this period.

INVENTORY
1. f. 36: [S]alve, mater gratie, stella claritatis / Dou way, Robin
2. f. 36v: Angelus ad virginem subintrans

So now you know. Well, I find that sort of wildly excessive information interesting, even if nobody else does....