Like Eivin said and you alluded to in your first post, there's line of sight blocker doodads that perform this function. They should block all vision, including air units. You may still get the issue where, if a player has units on two 'adjacent' planets and thus has vision on both sides, they might be able to shoot from planet A onto planet B even though the units doing the shooting don't have direct vision themselves. I'm thinking you can resolve that issue by removing pathing/terrain through the appropriate function, but if this is not the case you may want to repost your question in the data forums, as I don't believe there is any other quick-fix terraining-related way to prevent units from shooting from point A to point B if they have vision there.
@lonestarkiller: Go
Like Eivin said and you alluded to in your first post, there's line of sight blocker doodads that perform this function. They should block all vision, including air units. You may still get the issue where, if a player has units on two 'adjacent' planets and thus has vision on both sides, they might be able to shoot from planet A onto planet B even though the units doing the shooting don't have direct vision themselves. I'm thinking you can resolve that issue by removing pathing/terrain through the appropriate function, but if this is not the case you may want to repost your question in the data forums, as I don't believe there is any other quick-fix terraining-related way to prevent units from shooting from point A to point B if they have vision there.