Quoting Peter Ansell ansell.peter@gmail.com:
You would only be getting feedback from people who already knew about the spoiler issue, or from people who went to an FAQ asking about spoilers and were directed to the long-term poll. As you point out, a wiki insider started the only off-wiki/mailing list discussion about the issue that we know about. That should pretty much establish the issue as an insider, not outsider issue.
I think if the poll said something like "Wikipedia is conducting a poll about whether readers want spoiler tags in fiction. Click here to help out!" people otherwise not familiar with the matter would click and moreover would be able to give preferences about where they'd want spoiler tags.