I'm fine with just using multiple swipes.

There are only a few rectangular options possible for a given number of screens (which I'd love to be able to select from ):
1x9
3x3
9x1 (current)

1x8
2x4
4x2
8x1 (current)

1x7
7x1 (current)

1x6
2x3
3x2
6x1 (current)

1x5
5x1 (current)

1x4
2x2
4x1 (current)

1x3
3x1 (current)

1x2
2x1 (current)

1x1 (current)

...and I see no problem whatsoever with the diagonal screens wrapping around as the horizontal ones currently do. If they're that big a problem, then forcing users to go vertically then horizontally is a compromise, but one I really don't feel is required. I think a selector would be interesting, but once you have your screen locations memorised it'd be quicker to just swipe a couple of times, it's not like you'll be able to read any information on the reduced previews. Even more so if you had a large wallpaper that was spread over the multiple desktops (which I'd also like to do as I want 3x3).