Pretty easy to accomplish what your talking about with a simple switch mod. (Not a momentary switch)
Since most newer BIOS hacks will detect evoxdash.xbe FIRST, then look for xboxdash.xbe, you can install EvoX without disturbing the original MS Dashboard.
When you flip the switch off, the TSOP BIOS boots, and loads to the stock MS Dashboard just like an unmodded box. When you flip the switch on, the hacked BIOS on the mod will see EvoX is installed and boot that instead of the stock dash. If you're really paranoid, you can just boot off a CDRW with EvoX on it.
As far as "accidentally" going on XBL with the Mod on, your idea won't really work. Since you normally boot off an original game disc to play XBL, a MOD will boot it exactly the same way the TSOP would.
Just wire in a switch, and maybe a nice LED to indicate your Mod is on.
This post has been edited by WiLD: Jun 20 2003, 06:08 PM