So as mentioned in my last post, I've managed to make it work how I wanted it to! Check this out:
It's rather complicated, but here is a summery of what it does:
On spline segment id 1, there is nothing at all. That basically allows for wall segments.
On spline segment id 2, you get curtains, one each side of the window, a left and a right. They are randomised so that you get 1 of 7 types, and the left and right curtain is synchronised by the randomisation of the selector being set to X spline section. This seems to work, even though the spline is not separated at all the vertices; perhaps because of the "by id" specification of the generator? The curtains also have a random translation to allow them to be opened different amounts in every circumstance.
On spline segment id 3, you get 1 of two blinds that randomise. These use the adaptive setting to allow them to tile along the segment so that they don't become a really wide blinds on a really wide window. As the blinds are fixed models, there is a variation of height by moving the geometry in the z direction, to a maximum of the height of the blind height.
finally on spline segment id 4, you get a single curtain, on either the left or the right of the window (randomly), randomly selected from the 7 curtain types.
There is also a variable that allows adjustment of the blinds z offset, so that they don't become visible through a opening above, or through the ceiling/floor. This is independent for each blind type as they differ in height.
Finally there is an adjustment variable that allows the offset of the curtains on the inside of the spline so that they sit in the building correctly.
Basically, you draw around your building on the outside of the walls, making sure there are vertices at both sides of each window. Set all the IDs to 1. Then choose the window segments and change them to either 2,3 or 4 to set them as the above id descriptions. If you don't want a window to have a curtain, just set the id to 1.
Took a day or two to make work, and I don't think I could have done so without the critical piece of the jigsaw from itoo, but it now works.