I'm not sure, it would need to be highly manoeuvrable and I don't think ion drives are. As far as I know, they're currently more for providing gentle, cumulative and fuel efficient acceleration over long distances. For rapid deceleration and changes in direction they'd probably be hopeless.
I can't recall where I read it, but I thought that the plan was for a refuelling station satellite that could be resupplied cheaply. Presumably the robot caretaker would be able to refuel at this from time to time as it scooted about tidying up.