Ok, a little experiment later and...
Removed the electronics from the shell of the key and out of any kind of range of an RFID. The button was still able to lock / unlock / set the alarm. Therefore the radio for the remote lock/unlock does not need the little black chip to be there for it to work
Removed the black chip from the key and was able to open the doors, put the ignition on, but not start the car. Therefore the ignition lock must have some RFID-type device that gets a code from the little black chip and unlocks the immobiliser.
So, any new key shell will need to
1. Allow a radio signal out from key's transmitter to let the remote lock/unlock work
2. Contain the little black chip and allow a radio signal in/out so the ignition system can interrogate it and receive a code.
The RFID signal is likely to be very weak and therefore probably needs a plastic part to the body of the key to be able to work unless the black chip is presented on the outside, but as it's pretty fragile I doubt this would be a good solution.
For those who it means anything to, think iPad / old style iPod where there is a black plastic part of the case so wifi can work.
So we might be talking about a plastic shell with metal top & bottom facings or a metal shell with plastic parts at the front (key end) to allow the signals out.