There is a way of using another key along with backspace to go back by using the accelerators, but you would need to go into the code of the browser if you wanted to be able to go back with the backspace key. This feature has been requested a few times... so I'm sure it will be taken care of soon.
If you're interested in using a key combination to go backwards, ALT + LEFT is already implemented. To use a CTRL + BACKSPACE to go back, you would need to add the following to your Accelerators (Edit > Preferences > Configs > Accelerators):
CTRL VK_BACK = ID_NAV_BACK
If you would like to use a different combination of keys, here's a list of the names you would use for the keys:
http://kmeleon.sourceforge.net/wiki/index.php?id=ConfigFiles#accel
Brian