Also, I haven't read much on it, but wouldn't some sort of linear accelerator/rail gun be more efficient if you had a mining colony with the intent of sending a lot of materials back to Earth? E.g., you wouldn't need a complicated vehicle, just a container and method to propel it down a track. "Aim" for the right part of Earth, launch, and a couple days later, k'thunk in some desert.
That seems like it should be very practical, and quite possible.
I doubt you could do it just whenever just by aiming, simply because of logistics of pickup from random drop sites, and safety. But, perhaps once a week or even just once a month, you have the trajectories perfectly plotted based on the orbital position of both bodies and time of day, and have deliveries dropped at the same location all the time.
That complication that comes into play is determining what, if any, satellites and normal flight concerns you'd have. The informational network and computations that would have to occur to figure out the position of every satellite that may possibly come into the flight trajectory, and what the airlines are currently doing in the area of the final stretch of flight, might be rather insane. They'd have to calculate precise launch times, but I'm sure that could be done with enough frequency that the shipments are regular enough to clear the storage on the moon and get what is needed to the industries on Earth. That would be a very exciting system to really see in action and see what goes into pinpointing the launch times.