Well, we don't know for sure, but it is very likely that the "ow" is like rose, doughnut, toad, etc., for the simple reason that Semper Dowland Semper Dolens in its various forms has a nicer ring to it. Having said that, England was famous for its eye rhymes and off rhymes, the best example is perhaps Dowland's lute song "Come again", where "die" rhymes with sympathy and misery. Eye rhymes are when the word looks like a rhyme but isn't. Even then, there is no guarantee that what we now consider to be rhymes were not in the 16th and 17th centuries, and the other way round. Or that they were closer, or more distant. Lastly in regard to Dowland's name, diphthongs were much more pronounced at this time, of the type that one hears in Canada and the Netherlands in either English words or borrowed words--listen to the Dutch word for House as a good example. In the event that Dowland's name had a bit of a lilt to it, is would be an off rhyme with Dolens, however, composers at this time both Latinized and Italianized their names, so those affected variants might have made a perfect rhyme.
Another famous rhyme pattern is "Roi de Soleil"--pronounced in old French, it makes a very nice rhyme. Even this is the endless subject of debate--since regional variations were--and are--so widespread. dt To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
