Should be possible. The Xbox eeprom is 8 pins, only 4 of which are used (AFAIK). If you had a 3PDT toggle switch, you could use it to switch the 5+, SDA, and SCL lines from one chip to another. Soldering your I/O lines to the motherboard will be the toughest part.
Keep in mind tho that alternatively, all 4 of the required I/O lines can also be found at the LPC at pins 15 (5+), 14 (SDA), 13 (SCL), and 2 (GRND).
You could simply try connecting your slave eeprom chip to those LPC points and then boot. You make get lucky and find that the "LPC" slave eeprom chip overrides the master (on board) eeprom chip during the boot sequence, much like the way an LPC modchip (when detected) takes priority over the TSOP in the boot sequence. If this turns out to be the case, it would be a simple matter of disconnecting the 5+ from the slave eeprom before powering up the Xbox to boot from the master eeprom, or flipping a cheap SPST switch to boot from the slave EEprom.
These are all theories tho. I have never tried this mod (tho I might now), because electrically, it is possible if you figure out, and stay within, the unknown electrical limits, such as wire thickness, lengths, ect.
This post has been edited by xboxmods2977: May 14 2012, 09:27 PM