If someone gets as far as making a imagemagick wrapper where you can move sliders then hit submit and get a new image with the sliders in effect, I'll gladly write the JS needed to update the screen on the fly.
This already exists. Search for "MagickStudio" of check http://magick.net4tv.com/MagickStudio/scripts/MagickStudio.cgi for an example installation.
This is the closest thing I found to a project homepage http://studio.imagemagick.org/magick/viewforum.php?f=5