I while ago some guy in Italy posted a tutroial on how to do this, I followed it and it worked!
Instructions step-by-step: (translated from Italian tutorial)
1. It is necessary to join the 2 jumpers that will render the flashrom of the Xbox’s BIOS flashable. The tutorial for this can be found at: http://www.xbox-scene.com/articles/tsop.php (ignore the crap about being unable to use an LPC chip... that’s what everyone said back then).
2. When you have joined the jumpers it will be necessary to solder to point 3 of the right figure (commonly called A15) with a reasonably long wire: (downside of the motherboard)
This wire will need to go around the motherboard and outside the metallic casing.
3. This could be the moment to program our LPC chip using the special bios floating around the net called EvoX_MatriX_TSOP_flash.bin. This BIOS is essential for this task.
4. If you’ve been following the instructions on http://www.warmcat.com/milksop/cheapmod.html, you will probably have connected D0 directly to GND. You will need to have a method of disconnecting it from GND ˝ a second after a cold power up. Here you can see D0, it also exists on the underside.
If you have a switch installed, you could use this, otherwise replace the wire that is grounding D0 with a longer ~20 cm wire. Send the replacement out through the ventilation holes. If your having problems see the “Dealing with D0” tutorial for more info. Here’s how it could look:
5. As you see the D0 point is connected to the wire that goes to the metallic housing of the Xbox. Notice also the wire that we have connected on the A15 point under the motherboard (on the left and it exits from under the motherboard). These two wires exit from the casing of the Xbox as you can see in the following figure:
The metallic case of our XBOX will act as GND and we connect D0 and A15 to it. The point that I have taped to the case is the A15 point (the point under the motherboard).
6. You need to prepare a CDRW with the EvolutionX dashboard! This is required for flashing. Recommend latest 1.8.3285.
The BIOS directory of our CDRW will have to contain the bios that we wish to put in place the original TSOP BIOS. Remember that this time the BIOS file must be 1 MEGABYTE and not 256KB! Make sure you have all the flash types defined in evox.ini! These lines should be in the [BIOS] section:
Flash = 0xbf61,"SST - 49LF020", 0x40000
Flash = 0x01d5,"AMD - Am29F080B", 0x100000
Flash = 0x04d5,"FUJITSU - MBM29F080A", 0x100000
Flash = 0xadd5,"Hynix - HY29F080", 0x100000
Flash = 0x20f1,"ST - M29F080A", 0x100000
7. This is the most difficult step because the key maneuver that you will have to complete will have to be carried out quickly (The Matrix and the Neurosys circuit would do this for you). With the CDRW Evox inserted in Xbox and the D0 wire pressed with a finger to the metallic casing you boot the Xbox and after ˝ a second you remove the wire from the metal. Too late it will do an endless loop of rebooting, too early and you'll turn your Xbox off (because A15 is grounded). Important that you don’t let D0 touch the metallic casing after you’ve removed it. Now the Xbox TSOP will have to make the boot and it should load the EvolutionX CDRW. If it did not load you open the drawer of reader DVD and close it then it should load (make sure you’re using a slow-burnt CD-RW!!). Once Evox is loaded disconnect the A15 thread from the metallic casing.
8. Now you will be able to flash the TSOP BIOS from Evox since the LPC chip is disabled and the original BIOS enabled.
1 more question: Anyone know if I can still flash my cheapmod from evoX, I have some ideas but would like to hear yours b4 I screw up my Xbox.
Edit: Sorry about pictures, I just copied and pasted this text. See the Italian tut for pics:
Edit 2: I did step 3 first... I probably translated it bad