I've decided to do something similar in the Stareater. Marking an empty planet for colonization creates a colonization project and on colonization screen you can decide which star system will build colony ships for which planet. Once built, colony ships will fly toward to planet and wait until there is enough of them. When minimum population has been accumulated, a colony is established.
GUI could use more work but that would be a job for another time. I've been looking around and there is an interesting GUI library for OpenGL, it's not so well maintained but I'll keep it in mind for that sunny day when I'll be converting Windows forms GUI to OpenGL. Now next on the menu: space battles!