The problem is, merchants used as 'minor governors' will also be able to trade other resources, like elephants or wine. Even if the player chooses to roleplay and not use them like that, the AI will happily spam 'minor governors' and put them all over their fertile soils amd whatnot.
Hmm no clue if it is possible but if they spawn via script for the owner of the minor settlement(They die when it changes hands)-And don't make them recruitable and give them 0 movement points would that work?