Trying to understand the setup here.
The tailgate contains a mecahnical external button release?
It is locked electrically with a solenoid operated from the central locking system?
So the problem is essentially mechanical?
You need to re-create the action of pressing the external button and lifting the gate?
So you need a device as suggested by MarksDTM to 'press the button'. This will incorporate a remote switch to be mounted on the centre console by the handbrake, for instance.
You then need a lifter. An electric aerial won't do it, I'm afraid! Something to either push the tailgate a bit until the struts take over, or an extending lifter mounted instead of the struts.
To be honest, something like Omegatoy's low tech tennis ball idea may be best. Maybe some kind of spring loaded arm to help the tailgate up would work.
Are there any disabled vehicle websites or forums to get ideas from, I wonder?