I've been having a look around the mk5/6 Golf forums as the central electrics module will be very similar to the Leon's and also had a quick scan of the
mk2 Leon section on here but not found anything concrete.
If people have done it successfully there will be info on the net somewhere!
I'm not too sure about what you want to do with the Media System though. I've done the latest updates on my RNS 510 but it is different to the SEAT system.
I've done 'staging' which is the needle sweep in the dials on my car so that should work on yours too.