Well, I've been messing around with forcing the game to run triple-wide, and there are a few problems.
Problem number 1: 3D elements are vert-, so running it triple-wide actually shows you less of the stage background.
Problem number 2: The game refuses to run at any resolution wider than 16:9, selecting a wider resolution causes the game to default back to 1024x768 on restart.
Problem number 3: If you use SoftTH and force a wider render target (in my case 5040x1050), the game will render the 3D backgrounds wider, but 2D elements like menus and the fretboard are stuck in a little box on the left monitor. Checking the in-game display settings reviles that it still thinks its running 1024x768 even though it's bring forced to go wider, which is probably what's causing the misalignment.
Some screenshots of what I'm talking about:
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This is obviously going to need a patch to make it work right...