Thought I'd take this as an opportunity to resurrect two old threads having to do with "going up" to the parent folder:
Double click and Go up level
Up One Level
Either way, I think making dbl-click on empty lister space to be the best compromise for a few reasons:
a) it would be configureable, so for all those who have left the friggin ".." back in the past where so many seem to feel it belongs, there's something new here for them too 
b) it's not position-specific... whether we're suggesting a file display border button, a toolbar button, a hotpath item (highlighted as you suggest or NOT), or even the dreaded ".." item... all these ubiquitous methods share the requirement for one to have target in on a particular element of the lister in order to make use of it. Which is sometimes a big PITA when working with a sensitive mousepad on a laptop on a plane or a train .
c) going back to the configureability benefit... with the @keydown modifier in v9... we could configure numerous alternatives to just "go up"... we could have <ctrl+dbl-click> run open in dual, <shift+dbl-click> run new Lister, <alt+dbl-click> run new tab, etc etc.
This method just seems to bring more to the table for EVERYONE, as well as seeming to be a natural companion to one of the niftiest yet somehow trivial seeming features I love about Opus - dbl-clicking on desktop...