well by "poll" it's more of a thread. Once this deadline is up, edit the first post (or start a new thread) detailing the top 30 and giving people 1 week to rank their own top 20. #20 will be assigned 1 point, #1 will be assigned 20 points, etc...
At the end of the week, the totals will be posted with the number of points each person received. It's also helpful to put in parenthesis beside the total point number the total number of #1 votes that person received. And for the sake of completeness, you have to pick 20 out of the 30.
This first poll is not a bad idea, since it gives you a good idea of who is really good, regardless of their actual ranked score, and whether or not they are active currently. I'm willing to bet that there's a number of CTB players who weren't that well known but have more exposure due to this thread. So far we really do have the top 20 CTB players, they just aren't accurately ranked from #1-#20 (well #1-5 probably are accurately ranked based on your current criteria, but it'd be good to see if it changes in a 2nd poll). Then again, they probably never will be, but it's our best guess, right?
It's a bit more work, and you might not get people to vote again, but if the response is underwhelming, just make the video like you were going to make it before. You only lose a week, and it's not like there's a big deadline for you to get this done. Even then, you can work on the video now, just change the order once it sorts itself out.