This is a nice little medium between fullscreen and windowed, it'd be a great feature to add (I have 2 monitors)

i don't see why not, borderless looks alot nicer too.

supported

I would prefer to play this way, supported.

Okay, so here are my views on this suggestion:
You can just change the Window Width and Window Height. Granted it is not quite the same as borderless mode it is better then full screen with multiple monitors(I currently have 4 monitors so I know the pain of being thrown out of your game to reply to IRC or skype.)
So, I don't really support this, although it would be nice to have a bit more of a user friendly method of changing the game res. Maybe a drop down selection of presets and then also a custom option.

Also, maybe an option to have your mouse locked into the window.

The game would have to be moved off SDL first, it's pretty terrible at borderless fullscreen.
SDL can do it, but there's a pretty bad performance hit & mouse cursors vanish when not moving. Pretty useless really.

Would be nice, but yeah, it'll mean a big core change before it'd happen unless you hate yourself and want the above issues.