Finally!! After thousands of boot loops I got it working. There are several .diff's for implementing this mod in android.policy.jar, but what I forgot: this works only on a completely deodexed ROM and mine was only half deodexed. It is possible to run deodexed and odexed system apk's side by side, but you must not(!) run a deodexed framework with odexed apk's - this was my learning curve
Summary: If you are on a fully deodexed 4.1.2 JB ROM (I am on 91.2.26001 Retail.en.EU), simply copy the android.policy.jar to /system/framework, set the permissions to rw-r--r-- and reboot.
MAKE A NANDROID BACKUP BEFORE, just in case!!
If you want to incorporate this into your ROM (Omar?), I attached the needed smali's too.
If you are on an odexed ROM, here is a short help to deodex yourself: http://xdaforums.com/showthread.php?t=2179009
Enjoy skipping music tracks in your pocket by long pressing volume without turning your screen on!