Hey bitbutter -
Your solution obviously works, but i a LOT more work than it needs to be.
Here's how I'd do it (I replied from mobile yesterday so I didn't have access to it exactly).
1. Create 1 tk2dCamera.
Native resolution 960x540
2. Create the border using sprites. Add an 8x8 black sprite to your sprite collection, add a second material to the sprite collection, make it solid (important, for performance). Rescale the sprites as required to block off the 960x540 window. You want to rescale the sprites to form a fairly thick border around everything. Attach the sprites to the camera, so they always follow the camera.
3. In the tk2dCamera, delete the default override, and add a wildcard override - auto scale set to "Pixel Perfect Fit" (typo in my previous reply). If you were in tk2d 2.1, the equivalent would be "Closest multiple of two".
Thats it.
The advantage of this approach is you only have 1 camera to deal with, and you can do whatever you like in the borders - eg replace with tiled sprites, etc.