Canon sells focusing screens for the 1D series that have crop lines etched in, to show you where the 8x10 crop will be. Useful, but won't work on the 5D2.
For critical use, I have used gaffer's tape to mask the edges of my LCD on the back of the camera. (I did this for a 3:1 panoramic, but the process would be the same.) It doesn't help with the view through the optical finder, but with live view it works pretty well.
No way that I can think of to change the actual pixel dimensions in the camera. (Unlike, say, the Panasonic m4/3 cameras, which can do that.)