For seeing someone's source code to act as a commitment mechanism, you have to be reasonably sure that what they show you really is their source code - and also that their source code is not going to be modified by another agent between when they show it to you, and when they get a chance to defect.

While it's possible to imagine these conditions being met, it seems non-trivial to imagine a society where they are met very frequently.

If agents face one-shot prisoner's dilemmas with each other very often, there are other ways to get them to cooperate - assuming that they have a communications channel. They could use public-key crypto to signal to each other that they are brothers - in a way that only a real brother would know how to do.

Signalling brotherhood is how our cells cooperate with each other. Cells can't use cryptography - so their signals can more easily be faked - but future agents will be in a better position there.