This one wouldn't be too bad to code, since it's just defensive AI. However, a couple points:
- Destroying the spaceship. If it's in a spaceport, the only way I can think of to destroy it is to destroy the spaceport that it's sitting in.
If you wanted, you could also have a 'capture' scenario, for example a plot like "Plymouth has stolen your RLV. Your mission is to penetrate the heavily fortified Plymouth base and get it back." The game does have provisions for capturing an RLV, I haven't really looked into it much, but one of the last Plymouth missions you have to capture an RLV, so we can look at that to see what has to be done.
- Well, I'm assuming Plymouth would be defensive only, or would you want them to have units roving around the map?
So basically this one would be an easy one to code. If you wanted to have plymouth "patrols" it'd be a little more work, but not too bad.. if you start both players with mostly built up bases, you don't need to worry too much about researching with the AI player (so it doesn't appear to be cheating)