The Bb for the 6th course can be found in the late 17th-century works by the German composer, Reusner, if my memory serves me well. He used it as an alternative tuning, but mainly used Dm.
I have to take issue with one word you used: refinement. That term has a qualitative edge to it, as if you are saying, had earlier composers known of this tuning, they would have preferred it. It's impossible to substantiate that premise.
But most lute players - 99.99% of them - read tablature, and enjoy using the fingering, and therefore tuning, that the original composers used. Furthermore, baroque composers in Italy seemed perfectly content to keep using the old tuning right through the 17th and 18th centuries, even on the longer string lengths of archlutes and theorbos.