You imply this bounties are due to a honorable reason, "Mayor gets annoyed at being targetted all the time and having his workers ko'd. He sets the bounty". This way, the knights on white horses "A soldier (or duelist) from another town can then collect the bounty by subueing the ruffian".
A very good and beautiful story. A fairytale. But, why don't you think on the story told like this:
"Mayor of a town decided another town shouldn't evolve. So he decides to bounty hunt major players from there. Builder and money makers. So, since his town is evolved, the money in treasury don't count that much. So his members only contribute to bounties. And he puts the bounties and inprisons the guys from the target town and that town is parralized"
Well, why does your proposal has to be regarded in the beautiful way, with the halo on the top? It is a thing that will work in both ways, giving more advantage to towns with more money for bounties, and those aren't small towns, pissed by duelers, as those have fewer members, less money and even less money for bounties as they invest in town. Those towns with tons of cash will be those (almost) fully build, with maximum members, and less money needed for town development.
So this will be another way 2 help growing the gap between towns, between members.
just an idea: the bounty should be available as dueling is, for only some players. So, if a town gets attacked, the last attackers are available to put bounties on. Not everyone in the world. Just the last attackers, from let's say the last 48 hours. Pretty much a bounties linked to a reason, not a free bounty.