I guess you're saying how do we know pledgers aren't cheaters. This is a valid concern and trust does need to be earned.
Yes. The prisoner's dilemma is a dilemma because there is no solution without external context. There are in general two solutions.
- external punishment for cheating, which is why the mafia have a code of silence, and
- repeated rounds, in which you don't know when the rounds will end.
In terms of any particular community, they generally appeal to both solutions: the ability to fine and the continuation into the future.
I'm not saying I'm against a pledge - just that it is unlikely to work unless your pledgers have skin in the game, and can lose that value, or they are in it forever, and so is everyone else.