What about digging tunnels under the walls? You need a shovel, select a spot outside to start digging and then select a spot inside where the exit would be. Phlegm and time costs depending on the distance between the two. Possibly some items needed, like supports or lamps. Building it doesn't trigger crime debuff (assuming digging started outside claim), but using it to enter someone's claim does. Moving between two entrances would also take some time depending on distance, but slower than walking. Only one player can use it at a time. If braziers activate they can shoot at any tunnel entrance that is in their range, but it should take some time to destroy it this way. Entrance's HP should be visible to anyone. If one entrance is destroyed, and a player is inside - he will exit from the other entrance. If both entrances are destroyed then player is killed. Of course players can destroy entrances in standard way.
Protection from this, for villages, is to simply have claims extend far outside the walls. This way, tunneling would require you to start digging on claim, which would automatically give you crime debuff, and activate braziers. Noob camps can also use this kind of protection, but costs of making large enough claim would be prohibitive.