Regarding publishing, the only workaround I see so far is to target WebGL. You can use arbitrary resolutions for this.
EDIT: If deploying as Windows/Mac/Linux binary, you could just put some artwork on the extra spaces (this can be achieved as some background quad larger than the render quad). One could also use this area for on-screen controls if deploying to mobile devices. Perhaps making it look like the play area is inside an arcade cabinet?