>Would you comment on the example syntax I gave here?

I really have nothing against adding an exclusive notation for margins. But I think that the array notation for positions is only helpful in one case - when you have plain, relative coordinates like in your example.
- I don't feel that using nested arrays is the right thing to do when we want readability (Pos = [[500, -20], [23, -453]]). Especially if someone gets the idea to tag their positions.
- That way you would always have to give both coordinates. Currently, you can use the defaults (0, 0, 100%, 100%) a lot. I don't think I have many window definitions that define all four coordinates at the moment - especially if you ignore margins.
- You couldn't just do foo.X = 123; later in script. You would have to do foo.Pos[0]. And for that you'd have to make sure, that Pos is set: if (!foo.Pos) foo.Pos = [123, 0]; else foo.Pos[0] = 213;
The minimal gain we get in simple examples (Pos = [50, 420] instead of X = 50, Y = 420) does imo not make up the downsides coming from that notation.

Nevertheless, even with an extra margin-notation, I still favour the Left/Top/Right/Bottom notation. Not only because you have to do less manual math and at least I find it really comfortable to work with, but also because you can provide the scripter with sensible default values.