I haven't seen this rule posted anywhere else. But it's a game-changer for my Eldar, and I think several other armies.
A Psyker embarked on a transport still generates Warp Charge, and can still fire a Witchfire power from a fire point.
HOWEVER... this is the ONLY TYPE OF POWER a Psyker on a transport can generate.
So this means that now you can no longer bless either your embarked unit, or the transport itself while embarked on a transport. You've got to disembark first before being able to do either.
The exact text reads:
Psykers embarked on Transports still generate their normal number of Warp Charge points.
Psykers embarked on a Transport can only target enemy units with witchfire psychic powers,
and only then if the vehicle has a firing point that has line of sight to the target unit. Psykers
embarked on a Transport cannot attempt to manifest any other kind of psychic power.
Psychic powers cannot target units that are embarked on a Transport.