hello jim
i feel fairly strongly that at short to medium casting distances (60-110 feet to the fly), it matters less what type of cast, but more the power in the backcast that sets up the final deliver. thus, any cast (spiral single, snake roll, double spey, snap-t, single spey, yadda yadda yadda) can ultimately be made to curve, assuming other factors are not unfavorable (e.g. windy, lack of suitable backcast clearance, etc.)
to adequately make a curve cast with a spey rod (given the previous limitations noted on the previous post) requires the ability to overpower the cast at will, and also assumes that one has pretty reasonable loop control.
for a negative curve cast (the easiest), one can use any cast set-up, just come at it a bit more off-vertical, and don't hit it so hard. this cast might be useful, say, if you were right handed, fishing river right, to a seam behind a likely looking boulder, with fast water between you and the boulder. you might want to put a negative curve in the cast so that the fly lands downstream of the boulder, and the line lands upstream, in the faster water betweeen you; you could eek out a little longer fly "hang" in the sweet spot; if you cast straight, the current would rip the fly away from the sweet spot pretty quickly. this was a fun thing to do that i used in russia my first year there, and it worked pretty well.
i don't know if that answers your question...