[Zeroes000]

For something less GPU intensive, could this easily be accomplished by some png with a colour scheme, and a transparent bit for the 4:3 video to reside in.

Mame has been doing this for years, and the Pi can do this with bezels too, so I'm guessing that a PNG would be a better way to go?

Is this something that can be done within VideoPlayer, along with its settings too?

This could work for RetroPlayer too!

Also lends the possibility for skinners, and others to show off their borders.....

I would definitely still prefer to see a dynamic border option, as opposed to just static images. Especially if you still use a plasma or upgrade to an OLED, the static imagery would cause burn-in. Aside from the aesthetics, I would like the feature so you can watch 4:3 content on those types of TVs without having wildly different wear levels across the screen caused by pillarboxing. It might not mitigate the issue completely, but I would be surprised if it didn't help.