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You can as far as I know.

If you mean position on the steemit.com page, I think not. It's probably locked there as a featured post or sometihing. But only if using steemit.com. With other UIs that's probably not the case.

I was meaning as a way to measure support for proposals.
Downvote if you are against adoption.

There is no downvote against proposals. It was originally envisioned as a funding system and the way it works for funding is everything above the return line gets funded and anything below the return line does not.

Since it is now being used for polls, we have decided to promote a convention where there is one proposal FOR and one AGAINST. An AGAINST proposal was added for the 4-week change, so people who oppose the idea can now vote directly against it.

Good thinking, still seems simpler to use a post instead of a proposal.
Though that would result in autovoting conflicts, I'm sure.
Steem on.