I know I can change the HUD shortcut key by going into System Settings... > Keyboard > Shortcuts > Launchers > Key to show the HUD.
I need to do this from the command line, so I can put it in a postinstall script called from preseed.
The following seems to work for me (on Ubuntu 12.04) to set the HUD key to Alt:
I can replace
Note that if you have a different screen configuration (e.g. multiple monitors), the
and see where the