I feel that the current solution of "just put a link in the comments" is wrong for two reasons.
Firstly anyone digging through to answer old questions will stumble upon it as an unanswered question and may start working on a solution. Yes I know we should all read the comments but the fact of the matter when you look up the question it states "0 answers" which is false.
Secondly, and this is the more important case, when users are seeking answers and perform a search they too will see "0 answers". This makes the site less user friendly. Its bad enough that people tend to answer questions in the comments section, do we really need to force people to do it now?
Why wouldn't you allow from cross site answers when you already allow for duplicates. When something is flagged duplicate its not instantly deleted. Its flagged, a note is put on it stating its a duplicate an users can see see it and navigate to a solution. The current "solution" feels more like a work around for the fact that the site has a bug (sites can't communicate with each other), rather than being an actual solution. If more sites are going to be added with specific scope (see how there is vi.se and emacs.se and yet there are tons of so.se posts that already answered their questions), this really should be resolved much better. Otherwise you're going to have lots of duplicate effort or lots of questions with no answers.