They could maintain a public list of YC companies that abide by the code of conduct in order to encourage good behavior, and to help communicate to potential employees that they would get a “fair” option deal (as early investors, they would clearly have access to the terms of any financing deal).
[EDIT: they could also maybe publish a list of investors who have committed to following the guidelines, and consider excluding VC firms from YC events if they won’t commit to those standards]
Depending on how serious YC was about making a difference here, going against the code of conduct might even be grounds for exclusion from the YC community? My understanding is that YC has always focused on what’s best for the founders, even when they were the outsiders in silicon valley and at their own financial expense. Maybe now that YC is such an important player in the silicon valley ecosystem, they could use that power to maintain the health of that ecosystem in a way that few others could?