Yeah I know, that would be the only major problem with making a Gal Civ Dune Mod. However, through lots of research and study of the books, it is feasable to try and show the space side of the Dune books. It's there, it just isn't talked about as much. It wouldn't be a perfect mod just for the fact that it takes place in space... but it would be cool to see the politics between the houses in action. The books do have space segments, only a few sure, but we could build off of it very easily. The books focus on planets, politics, economics and ethics something Gal CIv does very well, therefore I think it's highly plausible to make an "Ultimate Dune Mod".
There might be sub-orbital clashes, but in the "iconic" Dune time period theres no inter- system space comabt, period. The Guild does not allow it, and no one defies the Guild. The Guild holds absolute dominion over Planetary Orbits (weather control) and intergalactic travel. Opposing armies are transported right next to each other in the same hold by the Guild to the planet, they fight it out, then leave in the same fashion they arrived.
Youd have to leave the iconic time period to the far future Dune universe of the Late novels, or to the non-canon fan fiction that Brian Herbert had ghost written, and just start pulling stuff out of your backside. In any event, the "authenticity" factor would take a hit.
Not to say it couldnt be done. A GC2 "intersteller outlook" mod, if it was authentic would be an entirely diplomatic, economic,political game with Dune planet titles, where you occasionaly purchase passage for troops to another planet...a lot of work for something that doesnt really highlight GC2's strongpoints. I just think youd be far happier with an engine that let you better accomodate ground conflict.
Ideally, a game Like M2TW would be a much better fit for a dune mod, with the geographical map replaced by a universe map, and the sophisticated tactical ground combat engine used to simulate planetary conflicts.