@Krelus: I have found a workaround, but I don't think you will like it

. It involves A LOT of tedious SEXP'ing, breaks the cutscene bars (at least on 3.6.10 RC2) and is really only suitable for ending a cutscene (i.e., if you want a long cutscene that ends in a black screen which then displays your opening logo/title/credit, this is a suitable solution. But if you want a cutscene that plays for a while, then snaps to black, then fades back in and continues, this solution will work, but probably drive you nuts in the process...) It goes like this:
A) Make a fade-out SEXP and add the fade out time, if necessary. It's probably a good idea to give FS some time, though, considering what follows...
B)Chain in a series of remove-background-image and remove-sun-bitmap SEXPs, that will amount to total removal of all background and sun images. This is the driving-you-nuts part, especially if you have a ton of background images. I had 35 in my cutscene and was seeing stars when I was done...

C)This is the important part: make a skybox that has a completely black texture and chain its appearance
after the disappearance of all the background images, with a set-skybox (or something similar, it's in the 'Backgrounds and Nebulae SEXP category) SEXP.
So, what you have done so far, effectively is this:
You have removed the background enhancements, ending up with a completely black background. Can you see where this is going, yet?

D) Chain in a series of camera positioning SEXPs, that will set the camera
really far away from the action, to avoid the remote possibility that a ship will fly across your beautifully black screen...
E) Fade back in.
F) Aaaaaand (drumroll) display your subtitle.
Works for me...But it's really tedious and it's just a workaround. So, yeah, dds and alpha support in subs is a must, I think...

One final note: Cutscene bars are not working properly if there's a skybox, or if the mission is a subspace one.

Keep that in mind...

Oh, and make sure and tell us if this will work for you...