I took the intro to clothes-making class. It goes over how to tweak the parameters on the stock clothing to get something you'd like.
There's more to it than that—the next level up involves building clothes out of prims—so I have much more to learn.
Even from the little I know, though, I think I know what would greatly assist SL tailors: a dress form, or more accurately, a mannequin. (You'd have one for men and one for women.)
The mannequin has to be adjustable in all the ways the body is adjustable in SL, with the same settings giving the same results.
The tailor should be able to work on the mannequin, and have the flat image that is wrapped around the mannequin generated automatically.
I think it would make it easier to produce clothing, and especially make it easier, for a given body shape, to get stripes and such to line up and to avoid discrepancies in cloth patterns where things are stretched to fit around the body.
That doesn't solve the whole problem, because it only works for a given set of parameters. You'd like clothing to adjust as the wearer fiddles with his or her appearance... but automating making the cloth pattern line up sounds like a difficult problem.