I had started to play with a multi-level priority menu soon after Chris revealed one on this site (the last incarnation before this), but I only got so far with it before other things took over http://fofwebdesign.co.uk/freebies-for-websites/css/priority-rwd-menu-multi.htm
It uses the checkbox hack to trigger sub-menus but there’s a JS fallback for IE8. The JS however should be forwards-compatible too, so, if you wanted to scrap the checkboxes, I expect you could strip them out and move the IE8 JS/CSS into the realm of the whole web page, without too much drama.
In the linked demo I don’t like how wide the dropdowns go on desktop, but like I say, this was just the start of something that I haven’t had time to work on recently. Maybe you could improve on that and share back?
I had pondered over how to handle the dropdowns for the most visible left-hand priority buttons, but the Aussie bank demo you posted has given fresh inspiration. Hope you can fill in the blanks and share your changes with me for incorporation in to the demo above.
Good luck
