It seems like such a common thing to do, that I was hoping that there was a standard template/function/whatever to instruct the stack to use (but don't own) a resource if it exists, otherwise create and own it.

That's pretty contrary to what pulumi wants you to do (which is why its hard). A stack owns a resource, meaning its responsible for creation & destruction, or it refers to an existing resource (in which case it does not create/destroy that resource).