> Anyway, I'm not sure whether the policy of SE allows suggesting redirections to non-affiliated sites. 

Indeed it does, particularly if those questions are egregiously off-topic for this site as you note.

> could we not show a note in bold, every time the "ask question" page is hit, that tells users to that if they have discussion-oriented or otherwise subjective questions, they should take them to gamedev.net 

We do have some specific help on the sidebar of the ask page:

> How to Ask
>
> - Is your question about game development?
>
> - We prefer questions that can be *answered*, not just discussed.
>
> - Provide details. Share your research.
>
> If your question is about this website, ask it on meta instead.

That also links to the FAQ and asking help page too: https://gamedev.stackexchange.com/questions/how-to-ask

I don't know if it'd be appropriate to speculatively link to a *specific* discussion site, but certainly we do try to make it clear that subjective questions aren't what we do here.