[PvK]

Two approaches come to mind:

1) In the editor, you could specify every location you want to be valid as a non-specific starting planet. My understanding is that these are chosen between randomly during empire placement, and that these are used before inventing new locations. You could then omit any unintended locations.

2) System types can be specified in the data files as being suitable for homeworld generation or not. This requires modding those files.

I'd test the theory you want to use on a small created quadrant with only a few systems before doing a lot of work based on that theory, though, but I think these are true.

PvK