Yes it can be absolutely done. Remember i'm doing this on FC6 using Gnome. You need to use Gnome Configuration Editor(gconf-editor). If you don't have it, install it by 'yum install gconf-editor'.
Step:1) Run 'gconf-editor' in terminal or from the link in some menu.
Step:2) Navigate to apps>metacity>global_keybindings.
Step:3) In the right pane, edit the value for any of the 'run_command_#' entries. Give it a value like F12,F10,<Control><Alt>e etc.
Step:4) Now navigate to global_keybindings in the same apps>metacity. Edit the value for the corresponding 'command_#' and type in 'halt -p'.
Don't get confused, for eg., if you gave F12 in run_command_1, enter 'halt -p' in command_1.
Now, when you hit F12 as in eg, the system will completely poweroff.
Some points worth noting:
* Entries can be edited by right-clicking on them and click 'edit key'.
* 'halt -p' is the command to completely poweroff the system. 'halt' will just shutdown without turning off power.
* Preferably don't edit any of the already assigned keys. Do a new one.
* Be very cautious while using the key(F12), no confirmation will be given and no open works will be saved.
NOTE: This will only work in Gnome using Metacity. Won't if you use compiz/XGL or AIGLX instead of metacity.
Hope this helps