This dirt cheap mod (~$15) will fix you up with a cool pedal that you can count on. You can quit fumbling for the select button and cursing when overdrive kicks in randomly. If you have a PS3 version, you might even get away without leaving evidence that invalidates the warranty.
This will definitely void your warranty if you are not comfortable with soldering. Get to know your instrument and be sure you want to goof with your warranty before you do this! Read up on Scorehero about the various Rockband controller iterations, you'll want to be familiar with this stuff before you risk your warranty.
Step 1: What to Buy
You'll need a pair of wire clippers, a hobby knife, or just a pair of sharp scissors will do for the little wire cutting we need. I also used a hot glue gun to set the wires in place. Solder is ideal though, so get your soldering iron out if you have one.
You'll also need a quick trip to radio shack for a 1/8" phone jack shown here. They carry these in the back of the store in the pullout trays full of little electronic parts. You'll find these in audio connectors. $3 will get you two.
Note: Most music gear comes with a 1/4" plug, but we'll need a 1/8" jack to fit into the space between the frame and the PS3 button. If you're going to drill a new hole into your guitar, feel free to use a 1/4" phone jack.
Step 2: Disassemble!
Be careful when removing the backing of wireless models. The battery pack is attached to the backing and you'll break that wire if you pull it apart in a hurry. As you can see in the picture, there's plenty of room to open it up gently though.
Certain models will also require that you remove the pick guard from the front of the guitar and tackle some screws there. Check here http://rockband.scorehero.com/forum/viewtopic.php?t=1902 if you've still got something holding it together to see if you've missed anything.
With the backing open as shown below; find the offending crappy tilt sensor. On my model here, it's right in middle to the right of the strum bar assembly. The wire connecting it all is going under the large circuit board on the top and into the top rightmost port. Click through to see the path of the wire if you're having trouble spotting it.
For this step, you can completely remove the cruddy tilt sensor. Just peel away any hotglue and unscrew the two screws holding it in. Cut the wires close to the sensor - we're going to use this long wire to connect our phone jack.
Note: there are some variations on the tilt sensor as well. Check the scorehero link for more info on those.
Step 3: Remove the Existing Button
Remove the little white button and bag it up with the crappy tilt sensor. If you ever decide to RMA this guitar and want to try your luck with the warranty, you can re-assemble it with a little soldering on the old sensor and some hot glue for this button assembly.
Step 4: Wire and Insert the Jack
It doesn't matter which wire you assign to which terminal; this is just an open/closed circuit - either will work. Just put one wire on the top terminal, and one wire on the bottom. If yours has a middle terminal (like mine in the picture) just ignore it - not used for this kind of connection.
Get that connection good and solidified then remove the ring-screw & washer from the end of the jack and poke the jack through the hole where the white button used to be. Re-attach the ring-screw and tighten it - no glue necessary - completely reversible! Notice here that the 1/8" jack just barely fits in the hole without blocking the use of the PS3 button. A 1/4" jack will not work here!
XBox users: your power cord will be in the way here. You will need to fine a new hole for this jack, and drilling one is definitely going to kill your warranty!
With the jack in place, re-attach the board over the PS3 button and close her up. You're done with the hard part!
Step 5: Pedal Recommendations
For sustain pedals, different brands may start either Open or Closed. You want one that starts open, and closes when you stomp it. Tell the guy behind the counter this; he'll know what you mean.
Also shown is the adapter to get the inevitable 1/4" connector you'll end up with on your pedal down to the 1/8" plug we setup. $3 at radioshack.
For your second guitar, I might recommend even tinkering with your pedal a bit. It would be cool to have both guitars plugged into the same pedal - one overdrive for both guitars! Use the same method we did for the guitar - open it up and wire in your own 1/8" plug.
Step 6: Done!
And admit it, it was kind of fun building this anyways. :)