One thing I've noticed about some stack exchange sites is that they have more detailed FAQs than we do (because they've been around longer). As part of these FAQs, they describe closely related, but off-topic questions and suggest other SE sites that would be more appropriate. Examples include the Mathematics SE FAQ, the Theoretical CS SE FAQ, the Physics SE FAQ (sort of), the Theoretical Physics SE FAQ, and the CrossValidated FAQ. What is the best way to get the word out to some of those sites to mention us in their FAQs as a destination for related, but off-topic questions?
I can definitely edit the Physics FAQ to link here. That has actually been a long-standing item on our meta.
update: this is now done.
Ask their moderators directly (in their chat room or meta), suggesting a line for the site.
For cstheory, I can add a line for this site to our suggestion list if you come up with a short description for it. See our FAQ for examples.
The FAQ of a Stack Exchange site doesn't normally refer another Stack Exchange site that still is in beta phase. The reason is that, being the site still in beta, it's not sure the site will pass the beta phase, and that usually a beta site doesn't have its own FAQ well defined, which means is still not well defined what questions are considered off-topic.
Even when the site is past the beta phase, the moderators of a site could need a certain number of questions that were asked, and closed as off-topic, and which could have been asked on Computational Science. When a reference is added to another site's FAQ depends from the moderators of that site, but asking to add a reference, and being able to show there is a real necessity, is already half the work.