Another thing to add.
http://forum.arcadecontrols.com/index.php?topic=50083.0
I did do a hack for an older mame to do this. But then they changed the artwork system and I didn't want to learn the new system. However it might be worth going back to now.
Also, adding a generic bezel. That way on games that don't have artwork you can fill in with something that matches the design of your cabinet.
I must be misunderstanding what you want to do, because I am certain this already exists. When I first set up my cab with v0102 I noticed I had artwork on the sides (1942 for example) and I hated it. It was making me mental. Turns out it was just a config file setting in the mame.ini, so I flipped the value, the artwork went away, and I had empty space on the sides again.
I can't remember which thing I need to set to get rid of them, but here is what the doc says (I think it was either the artwork flag or backdrop flag):
MAME core misc options
----------------------
-[no]artwork / -[no]art
Enable or disable usage of additional artwork (backdrops, overlays,
etc.). The default is ON (-artwork).
-[no]use_backdrops / -[no]backdrop
Enables/disables the display of backdrops. The default is ON
(-use_backdrops).
-[no]use_overlays / -[no]overlay
Enables/disables the display of overlays. The default is ON
(-use_overlays).
-[no]use_bezels / -[no]bezel
Enables/disables the display of bezels. The default is ON
(-use_bezels).