Python or dll but not xml.
In Python, I guess that on the first turn you could check for Settlers and issue a pushMission for the MISSION_FOUND.
If it's for a FfH2 Mod, I think that you have excellent modders/coders in that part of the forum that can help you. I'm assuming this because you mentioned in the Quick Modding Questions thread that the problem was that Settlers had an increased visibility which made the AI postpone their decision to settle. In plain BtS, I think that the AI almost always settle on the initial spot. In a sense, it would be best to just give back to Settlers their normal BtS visibility!