2 options.
1 - you can modify the tk2dCamera matrix to be off center, or do some manual offsetting while scaling
2 - You can switch the camera to a normal camera and set the ortho size correctly to display 1:1 at any resolution. Basically, the ortho size needs to be 0.5 * screen height. A really simple script which sets this on Start() will do the job. You can still keep a second tk2dCamera for any UI elements, just make sure you set depth to be bigger than the first camera, and set clearflags to depth only or none.