I have an 2008 Sequoia and just put 5.1 firmware on my 7.1 disk. It works.
I used a Memorex DVD+R DL and burned with Roxio Basic using no special settings. The key-sequence for override works and the full features become available at any speed.
Two downsides:
1. You need to override it each time you start the car.
2. You get that annoying warning message and have to hit OK - EVERY time you drive the car. This might be worse than simply giving up the ability to input
I am thinking a hardware mod is going to be worth it unless someone can hack out that warning.
1) Have the 5.1 KWI ready
2) normal copy 7.1 disc into path let say c:\temp\
3) replace 7.1 KWI with 5.1 KWI
4) use NERO to burn a dual layer disc of all files in temp directory
I don't need to do ISO, replace file in ISO, burn ISO?
Just got my 7.1 disc a few days ago..now I just need to buy a dual layer burner.