I had an idea. I was thinking of putting the search form below the list of posts, the <dl>, and I considered using absolute positioning. I thought of two ways to make it work.

When you adapted Kroc's CSS to make this style, you used his properties that set the body height to 100%, the original purpose of this is to allow child elements of the body to have a min-height of 100%, but this doesn't seem terribly necessary for the forum's design. So if the body's height is set to auto, you can absolute position the form element to below the thread list:

body { position: relative; height: auto !important; }

form[action^="http://google."] {
position: absolute; bottom: 815px;
left: 5px; right: auto !important;
}

I also thought of a different method of positioning the form to below the thread list, it looks something like this:

form + dl { position: relative; z-index: -1; }
form[action="google.com/…"] { display: run-in; position: absolute; bottom: 0; left: 0; }

I haven't tested it, but in theory it would cause the form to "run in" to the <dl> element, which would become its new containing block, then the <dl> element's position is set to relative, and the form could be absolutely positioned within the <dl>. I've had this idea before for other things, but I've never actually tried it and I've never seen it done before. (Technically, once the form run's into the <dl> it would become an inline element, however, this doesn't matter because all absolutely positioned elements can have block properties.) Unfortunately, I don't think Firefox supports "run-in" properly.