Should use the ISS as the base mass for a EM launcher. Seems nuts to waste so much on that existing potential energy.
ISS is in a bad orbit for this sort of thing: halfway between the best orbit for Baikonur and the best orbit for Kennedy, it was a compromise which was okay because it wasn't getting that much traffic, so the penalty didn't amount to much at the scale of the budget. Using it as an orbital way-station will end up paying that compromise penalty a whole lot of times on all the launches up to the station, driving up the total value of the orbital penalty. If you are doing something big enough in space that you can afford to keep the ISS in operation (1) then you are probably better off building your own platform in the right orbit for your launch site. (2)
Also, repeated dockings and undockings (as you assemble the mass driver and then use it) will ruin any microgravity experiments you might want to run, so you can't really combine "base station for deep space exploration" and "science lab" missions, you really need the station to be focused on one or the other.
1: The main problem ISS has is political, not technological. By design you need the US and Russia to cooperate to keep it operating, and that made a lot of sense in the 1990s, and has fallen apart over the past decade.
2: Orbital plane changes- to move ISS to the right orbit for a specific launch site- are hideously expensive in LEO. GEO satellites often find it cheaper to raise their apogee out to near the moon, make the necessary plane change way up high, and then drop the apogee again when they get to perigee, rather than make the plane changes in LEO. (ISS can't do that.)
It's in the ideal orbit for Baikonur. It's much, much easier for a low-latitude launch site to reach a high-inclination orbit than vice versa. This is especially true for Baikonur, where the available inclinations are restricted by human habitation and foreign countries downrange. Hence the minimum inclination is 51.6 degrees (which is where the ISS is) despite the launch pads being at 46 degrees latitude.
You are correct, I had remembered the details wrong- ISS and Mir and Salyut 7 all have the same orbital inclination, so that's clearly the best orbit for Baikonur. So if you were doing your mass launches from Baikonur you would not pay much of a penalty. From KSC, though, you'd almost certainly want a lower inclination to get more payload up.
Really, you pay the same penalty launching to 51.6 degrees regardless of whether it's Baikonur or KSC. But you're totally right, you can do even better by launching to lower inclinations out of KSC, which can go down to 28.6 degrees.