I tried installing this - somehow it refuses to work - my showthread template is quite modified, and so is my postbit - but you would expect it to work.
The main problem is that when I click on the submit button nothing happens. I suspect that I am not getting the form tags in the right spot - initially I thought it was the "Thread Rating" form tags interfering, so I took it out - didn't help. Then I thought maybe I am not placing and closing the form tags in the right place, so I put the form tags just above and just below the $postbit variable with the input line between them - you would expect this to work for sure, but it still didn't.
Anyway, I don't really need this hack, so I have given up.
For those who want the TEXT version, I have attached it below (exported it from vbhacker):