[bendhoefs]

What booster would you use?

Another booster up in the fairing? That would need to be quite heavy. It would be a totally custom thing for this specific mission. You would need a suitable storable propellant.

Leave the booster from the Ariane attached? The lh2 and lox would boil off after a few days.

What do you do if the deployment fails? SpaceX's dragon can't do space-walks on its own, it doesn't have an airlock. There would be no way to fix it short of developing a whole new space craft for that task.

[samwillis]

For space walk accessibility maybe they should have docked it to the ISS for initial setup before then boosting it to its final point?

[zrail]

ISS is in a very low and eccentric orbit. I think boosting from ISS to a geo transfer orbit, let alone escape, is more expensive than just going straight to escape velocity.