An API for Orbit objects, Planetarium objects...

Currently you can do that to your orbit with:

mechjeb.core.part.vessel.orbit.getPositionAtUT()

or to the selected target with the Rendezvous module with:

mechjeb.core.targetVessel.orbit.getPositionAtUT() (for Vessel's)

mechjeb.core.targetBody.orbit.getPositionAtUT() (for CelestialBody's)

I'll try to add a function to get the orbit of an arbitrary target to help a bit too.