It appears I missed a trick on the Lunar fueled post. If two units of volatile mass are launched from the moon to sling around the Earth in opposite directions, they can theoretically impact each other with a closure rate of 22.4 km/s. 11.2 escape times two. If they impact behind the pusher plate of a vehicle in sub orbit, they release energy equivalent to a rocket exhaust of 22,400 m/s, or an ISP of 2,285. Since only half of the material would impact the plate, the Isp would be half that before subtracting various efficiency losses. A net of around 800 seems likely with a water based volatile impactor.
The advantages of this are that the impacts could follow the vehicle all the way to orbital velocity without losing much Isp. And all the Isp comes from Lunar material with no need to carry on board propellant. Other than the pusher plate, the rest of the vehicle is payload. A ton of Lunar fuel provides the energy to orbit a ton of payload.
Clever orbits would seem capable of exceeding escape velocity for outbound vehicles.