CPWizard on an arcade monitor

[Neverending Project]

I made a custom layout for CPWizard where the background is 320x240 pixels and set everything up on a friend's MAME arcade machine which has a K7000 arcade monitor. The front-end runs at 320x240, but the games switch to various resolutions. When I hit the key to show CPWizard, it dutifully hides MAME and displays my custom layout, as expected. And it appears to change the resolution to 320x240 as well. When I exit the CPWizard layout screen it changes back to the game resolution, but there is an artifact of the front-end that can be seen around the edges of MAME.

I may not be explaining this very well, but I am wondering if this is because CPWizard is changing the resolution to and from 320x240, or maybe it is an issue with something else? I imagine one solution would be to hide the front-end when launching MAME so only the black desktop would be visible, but I don't think this is possible with MaLa. Does anyone have any suggestions?