L’amico @brusarp, che ringrazio, mi ha informato che è disponibile, su SamMobile, la leaked 4.2.2 per Samsung Galaxy S3. A seguire le istruzioni ed il link.

Firmware Details
Android Version: 4.2.2 – JDQ39 (Jelly Bean)
PDA: I9300XXUFME3
CSC: I9300OJKFME1
MODEM: N/A
Region: Middle East
Carrier: Branded
Changelist: 611730
Build Date: 8 May 2013
Download: I9300XXUFME3_I9300OJKFME1_ILO.zip

Flashing Instructions
– Unzip the file
– Open Odin 3.04 (already included in the firmware package)
– Restart phone in download mode (Press and hold Home + Power + Volume down buttons)
– Connect phone and wait until you get a blue sign in Odin
– Add I9300XXUFME3_I9300OJKFME1_HOME.tar.md5 to PDA
– Make sure re-partition is NOT ticked
– Start flash, sit back and wait a few minutes.
– If you encounter any issues with the firmware (Any FC, Bootloop etc)
– Boot into recovery mode (Home+power+vol up)
– Choose to wipe/factory reset. (THIS WILL ERASE ALL OF YOUR DATA INCLUDING YOUR INTERNAL SD CARD!)
– Then choose reboot and you should be good to go!