Hrm, can't think of a good place to put it, but maybe
this will work for you.. Commonly called a jump menu,
or jump box.
You can find code snippets here
which I believe I have used before
Then, near trhe bottom of /include/line.cat.php , locate the line
$htmlsrc .= "</td></tr></table>";
First I changed the surrounding double quotes to single quotes,
just to make things a bit easier, then added another TD pair
to contain the form . . . .
$htmlsrc .= '</td>
PASTE FORM SNIPPET HERE, after correcting the URLs of course
Since line.cat.php is only included when viewing a category page, the jump menu wont appear on the main page, or the whats new/cool etc pages.