Hi,
you can change your dashboard from unleashx to xbmc if you change the default.xbe file in the e:\dash folder. You might create an unleashx folder in your apps folder and copy the e:\dash files to that location. Xbmc comes with a shortcut xbe. Using that, you don't have to move the complete xbmc package to your dash folder.
changing the eject button application is a bit more tricky. The application that is booted is determined by the nkpatcher program. That one uses a configuration file with all the settings that are adjustable. The older versions of that program needed to be recompiled if you changed the settings. The latest nkpatcher versions can read that configuration file at bootup.
Usually, the nkpatcher is on the original c partition (not the shadowc). In my Ndure setup, it is in the bios folder.
Experimenting with nkpatcher is dangerous. If something goes wrong, your xbox won't boot anymore. The only solution for that is connecting your xbox harddisk to your pc, and undoing the latest changes. You need your xbox eeprom.bin file on your pc for that. So, make sure you backup that one before you start.
Even changing the dash can cause similar problems. You better test xbmc as an application first. Than you can test out the shortcut and it's configuration file from a different folder. If that works, you can copy the 2 files to the e:\dash folder as a replacement for unleashx. Better a couple of steps to much than ending with a non working xbox.
You can always start your unleashx dash from within your xbmc one. A dash is just a xbox application the same way a game is. (If you copied the unleashx files before you switched the dashboards)
regards.