We're replacing the sink and vanity with a 31 inch model -- and now of course the light won't be centered anymore. So, I need to either move it 3.5 inches to the left. I wouldn't have to cross studs, but the existing box has wires coming in from the top, and both sides, and I don't think I have very much slack.

I realize one option would be to use existing box as a junction box, but then I need to keep it accessible == ugly.

Another option would be to buy a "3-wide" sconce, and route the wire through one side. However, that limits the available styles (a lot of 3-wide lights still have one 4" attachment in the center).

So, I was thinking -- could I make a wooden "bezel" that could cover the existing box? i.e. a 1 foot wide, 4" tall, 1 inch thick board. the light fixture would then get mounted to the wooden board. Wires would go to the backside of the board, which would have a channel routed, such that final connection is within the junction box. Alternatively, I could install a second junction box next to the first, so that all wires are run behind the wall, but the wood board would cover both.

Would such a thing be legal? I guess the real question is whether you can use a wooden bezel as a decorative cover to a junction box, or whether it needs to be plastic/metal? Or, if I use the two-box idea, and cover the original box with a metal cover, can I cover this up with a removable piece of wood.

Unfortunately, that's one of those fuzzy terms that potentially allows for various interpretations by a code inspector. Nevertheless, I'd think that if the cover (whether board or metal) can be easily removed by a couple of screws, you'd be on fairly solid ground. No slam-dunk case, though.