I don't think anyone has figured that out. I know all trade buildings do a certain amount of local trade but it is nowhere near what inter-faction trade does. Although MTW has been around so long I figure people must have tried. If thats the case then the answer it probably can't be done. It could be hardcoded.

The thing I do know is WesW somehow figured out how to trade overland. I am not to sure how he did it but using an agent he created called merchant he somehow figured out a way to get landlocked regions to trade. That is in Wes' MedMod series.

I know VikingHorde fiddled with the Agriculture to have Provences produce roughly 30% more money from farming. Thats in XL.

I know you can add trade goods and adjust the prices of revenue trade goods garner, although I've never done it myself.