I agree with Thomas on all points, including that cary should feel comfortable acting as a community member (being clear to distinguish advice as such).
There are two separate issues bright ou here: * the issue of communicating a guideline or rule of thumb to all projects (ways to have a cross-wiki discussion, about vandalism or checkuser or other; which can be discussed on Meta as a starting point), and * the issue of having cross-wiki policies (we do have a few, but it's not that easy to figure out what they are or to propose new ones)
SJ