I dunno if you ever heard of / played shadowbane, darkfall online, or any number of similar games (even other minecraft servers), but if there is some way to possibly grief someones crap, no matter how big / active your town is, a group of people will log in at 4AM in the morning and grief your crap lol. I have no idea how the tnt cannons will work, how effective they will be at getting to chests etc, but i feel the way this thread discussion is going, they are either gonna be practically useless, or result in everyone building obsidian structures or tiny boxes in the bottom / center of town to abuse region limitations on tnt placement, both of which are not fun options to me.
IMO, Tnt should be able to be placed anywhere, even in regions, allowing raiders to get anywhere in a town and grief / take over peoples towns as long as they can hold out. However, chests should be protected in some manner to prevent true griefing. I dont know if this is even possible, but maybe "chests that have not been accessed in last 5 minutes (random number, adjustable)" would be protected completely from any access to anyone not part of town / guest; HOWEVER, to give raiders some economic incentive, any chests accessed in last 5 minutes (not lwc) could be opened / raided from. This would prevent people hiding in towns from quickly stowing all their stuff when being raided, and raiders would at least get the equipment they had on them if successful. Just an idea, probably not even possible to be programmed, but its an idea /shrug.
Personally, i just really dont want this mod to result in one optimal town design specifically for the purpose of taking advantage of the limitations of the tnt / region limits. Then all our towns would look the same and that would be dumb.