Disclaimer: This worked for me on my Droid X. You might end up ruining your phone and even hurting yourself in the process. Do this at your own risk. I am NOT responsible!

1. You will need a spare USB cable (this will be cut).
2. Cut the end off opposite the rectangular computer plug side.
3. Strip the red and black wires. Red is Vcc (+5) and black is ground. Get the 2 other wires out of the way (I just cut them off).
4. Remove the battery and you will see 4 contact pins on the phone that the battery contact pins goes to (it flexes up and down).
5. Hook the red USB wire (+5) to the far left pins (towards the camera flash).
6. Hook the black USB wire (ground) to the far right.
7. Make sure the red is not touching any other metal except the pin. If necessary, secure with electrical tape or put in the battery.
8. Verify the connections by looking at the polarity label on the battery itself.
9. Plug the USB into the computer and that should solve the battery problem.
10. Plug the USB data cable to your phone, then you should see a message similar to this, “Ready to program, connect USB data cable.”
11. Reflash/sbf or do whatever you need to do.
12. Make sure you unplug the cable from the computer before removing the wires from the pins.

Basically, it's just bypassing the battery.

• http://forum.xda-developers.com/showthread.php?t=892026
• http://modmymobile.com/forums/8-guides-downloads-forum-suggestions/64614-how-flash-phone-battery-low-cannot-program.html