Can I rotate a WorldEdit selection horizontally?

Yes this is possible:

//deform rotate(axis1, axis2, degrees)

So in your case, I think you want

//deform rotate(x,x,90)

I haven't got it installed at the moment, so unable to quickly check.

Another suggestion is //deform swap(x,y)

At the moment, Rotate command only rotates around Y axis(Or as you said, Z, depending where you draw them).

However, in here: http://redmine.sk89q.com/issues/633

It is suggested to use either

//deform rotate(axis1, axis2, degrees)

or

1. Select a cube containing your blocks. (height, width, depth on //size should be the same)
2. //deform swap(x,y)

Sources:

• http://wiki.sk89q.com/wiki/WorldEdit/Clipboard#Rotating
• http://redmine.sk89q.com/issues/633

The bukkit command is "//rotate (y axis)" by default, but you can go further by "//rotate [(x axis) (z axis)]" so I rotated mine exactly like in your picture by typing in "//rotate 0 90 0" although you could spin it any way you want with that.