Unless there’d be a timing issue having the border generated elsewhere in the chain, it could be as simple to implement as adding a toggle to the firmware to change the cropped border color between black and white. Even better if there was a user-friendly submenu with a toggle that could enable the cropping in the correct color and included a thickness adjustment that would adjust all edges simultaneously (and perhaps enforce a minimum thickness, if necessary).
That mask is probably the one problem I have with the Sinden Lightgun setup. Is that white border displayed on the screen out of convenience, or is it relying on the backlight strobing for something, or perhaps to match the TV’s brightness?
What I’m wondering is if, for those that don’t want to have any part of the screen cut off (because the processor ostensibly isn’t scaling the image, just cropping the edges), would it be possible to tune it to use something attached to the display’s bezel? EL tape might be expensive and be a bit power hungry, but it could be cut to size. Long, thin acrylic strips could be lit with just a couple white LEDs (perhaps even just two, top left and bottom right), but they’d be harder to ship and/or cut to size. Assuming it’s bright enough, a single one of these $12 EL wire kits could fit around a 65″ LCD, and could be cut to fit anything smaller.
Also, in the demonstration video on the linked article, the display was set to stretch the PS1’s 4:3 output to 16:9. Has it been demonstrated to work in the correct aspect ratio for those consoles?