Yes, that would work, though its javascript rather than css. Same trick though. I think all the needed code is here:
http://www.leefish.nl/mybb/thread-1330.html
I added a link in the sandbox - borrowed your image there Rocketfoot . wave your mouse under the picture in the first post and you will see it. That pixel.gif is a default image in mybb, its in the images folder of the default theme.
What I did notice is that it is slow, even on a vps. So.
I tried using it on a post button. No luck so far....