I would assume you could simply expand grav wells out far enough to touch, that way you could make a direct course or go to the edge and jump. I'm not 100% sure how you would do that (the engine can obviously handle it, the low grav bonus shows that) though I'd assume the PlanetBonusLowGravity.entity file might be a good starting point.

Good luck

The program uses three things to determines the edge of the area of an planet. First, the radius where you can build platforms, second is the grav well, and the third is the full radius of the planet area... this area makes possible to stations your carriers outside an Magetic Cloud grav well, to launch fighters. This region is normally 20-30% larger than grav well. To makes ships capable to travel between planets in STL you need to expand this third value enough to reach other planets radiuses. As i know, the game haves limitations for that values.
Now a capital ship has a maximum sub-light speed is 500 units/s and the phase travel got 29500. Interstellar travels have an 24 times multiplier for that speed. So... if you need 15 secs to reach another planet in phase, than you need 14 minutes and 40 secs in STL which makes a sence. For compare... in the built in map designer, you can change the lenght of phase lanes, if you set this to the maximum, than a ship is need 10-20 minutes to travel between planets... in this sceneario, in STL speed a trip takes 9.8 hours to 19.5 hours.