Re: BEAM - lack of ports
"Presumably they could have more than one, as long as they only used one at a time. Not that another robot arm is a realistic prospect of course, but power wouldn't seem to be the problem."
Not possible in the situation described as you'd need to be able to do a hand-off, meaning you have to co-ordinate the two arms, meaning you'd need them both operating. I don't think one can rapidly switch power between the two in such a scenario, plus there's the risk that turning one off will allow it to drift, making alignment more difficult.
In any event, Canadarm2 (the arm aboard the ISS, officially the MSS) turns out to be self-relocatable, so it could maneuver itself into a port extension. But like with a tangle of USB hubs, I'd feel a little uncomfortable about the logistics.