You can specify a "base location" by placing a player's start location in the middle of nowhere.
The AI script will expand to that location normally regardless of mineral/gas content near the location.

This is because Starcraft will insert Start Locations into an array of base locations.

So the AI will attempt to expand to the spot where its Start Location is, regardless of what's in the area?

Interesting tidbit. I'm sure this can be made use of.

It can be any player's Start Location, doesn't need to be its own.

Does it also use these "base locations" when it determines how it is going to use it's army to attack?

I don't think it does.