though this thread is a bit old, I just add one expression based on the ones posted above (thanks a lot to everyone here as I have no idea how to write this sort of thing!! 
)
if like me you don't want to have your folder name modified (cuz you have other info) and just want the bitrate added to it
e.g:
Artist - Album - year [Japanese Edition] (remastered 20XX)
becomes
Artist - Album - year [Japanese Edition] (remastered 20XX) [320]
choose: Format value
field: _directory
format string: %_directory% '['$if($eql(%_vbr%,'VBR'),VBR %_bitrate%,%_bitrate%)']'
it worked for me 