You can't deny that Hotseat is a form of multiplayer.

And the points I'm trying to make are for multiplayer in general, not just networking. There will be differences between different kinds of multiplayer, but simply introducing the concept to a game can require mindbogging amounts of work.
I agree that Hotseat can be a very enjoyable experience. I've played HOMM 3 many times with my brother, laughing ourselves through and past 2am; but you're probably aware of the problems Hotseating had even there. "Teal sends you resources..." who did it send them to?

Still, it greatly extended the game's lifetime, and I'm confident it would do exactly the same with GalCiv II.
But this is for people who would otherwise play on a LAN. This is still a significant amount of the multiplayer population, but it's also considerably less than the full population.
We're already aware of a minimum of code changes involved. They will be much less than when true networking is supported, granted, but they will still be considerable. Diplomacy is the most straightforward concept that will need changing, but also the concept of stances in general. Are you friendly towards someone or not? The computer cannot determine this for you anymore. The Diplomacy bonus for your race may effectively become completely useless! The Galactic Bazaar (Project) can be thrown in the dumpster as well.
Stardock worked very hard to give AI the feel of being as lifelike as possible. Their implementation makes it easier to believe that the AI are "full-fledged" players; although they follow the same rules as any other player, they are not truly human players in those respects. I don't know if I'm prepared to lose some of these pretty unique features in exchange for multiplayer support.