I've had success when dealing with dropboxes like this in IE6 with just setting the selectedIndex
property on the parent select
element, rather than trying to tell an individual option to be "selected".
$('select#mySelector')[0].selectedIndex = 3; // or whatever
You could determine an individual option
's index by just counting his prev siblings
var si = $('option#myOption).prevAll('option').length;
It was interesting to see in the second link you included, that a solution was given suggesting you inject a slight delay between manipulating the DOM and trying to do what you want. I'm guessing IE6's rendering/javascript engine must rely on the element being visible on the page before allowing this action. (The settimeout would allow this occur.)
Good luck!