e.g. I wrote some SLIC code to select the biggest army the AI (of one of the civs) has, and to make it move to a particular location. The selecting part works (I put in an appropriate message to indicate that), but the moving part of this doesn't work.

I haven't messed too much with this yet, so I could well be way off, but I think that if you want to override the AI completely, you'll have to work very, very thoroughly. Don't just send the AI to it's final destination, send it to an adjacent square first. If it has movement points left, send it to the next square, and then to the next, etc. That will probably work better. Also, there are a lot of ways to move the AI (much be like 5-10 events that are movement related), so trying a different event might help.

If you get stuck, send me what you've got and I'll have a look at it, I'm currently working on a more or less similar thing for a scenario anyway (restricting movement to a certain 'zone' to be more precise, not quite as complicated but still similar), so together we might be able to work it out.