personally i can't stand navigational elements jumping around. most sites i revisit i don't even wait for them to load b'cos you now where the link you need is (who actually reads words on a web page till they're at the content they want?).
however, i like your idea and for some sites it would be beneficial. how about checkboxing the option within each navigational group so that it can be turned on and off.
that way, main site nav can be properly structured but things like products lists can arrange themselves.
nice feature, need to be customisable.
good luck.