As a longer-term solution, I would suggest adding cross-posting as an explicit reason when flagging and/or closing questions. This is already being used on [Unix & Linux](http://unix.stackexchange.com), listed under *Off Topic*.

> This question has been posted on multiple sites. Cross-posting is strongly discouraged; see the [help center](http://unix.stackexchange.com/help/on-topic) and [community FAQ](http://meta.stackoverflow.com/questions/64068/is-cross-posting-a-question-on-multiple-stack-exchange-sites-permitted-if-the-qu) for more information.

It seems to work well. Off-topic, cross-posted questions are generally closed soon after I flag them as such.

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In the short-term, this suggestion doesn’t help those of us (such as the OP and myself) who have less than 3000 reputation and can only flag questions but I think it would be useful to add this feature.  Note: It would also involve up-dating the [What topics can I ask about here?](https://serverfault.com/help/on-topic) help page.

Here’s a non site-specific version that might be useful for custom close reasons:

> This question has been posted on multiple sites. Cross-posting is strongly discouraged; see the [community FAQ](http://meta.stackoverflow.com/questions/64068/is-cross-posting-a-question-on-multiple-stack-exchange-sites-permitted-if-the-qu) for more information.