A little update on the tests I have done so far without success
At least I hope it will save time to other trying to achieve the same.
- Retrieved the framework-res.apk file from the stick (in system/framework) via ES explorer in root mode (could also be done with adb)
- Used apktool to uncompile the .apk
- modify two values in "res/values/bools.xml" file from the uncompiled .apk :
Both were set to "false" and I changed them to "true", it's mainly trial and error method as I don't know the exact underlying behaviors ..
- repack the .apk with apktool
- replace the framework-res.apk on the stick with the one modified
- install a few "rotation app" from the market to try to force "portrait" mode
None of this worked
Any clue ?