I found a script at the GIMP Plugin Registry "rotatebrush.scm" It both resizes and rotates a brush while you are using it, sort of. Well close enough to make me say Yeah man Yeah!!http://registry.gimp.org/node/16231
use your terminal and sudo into your file manager - mine is sudo thunar, then password
go to your home folder, then the hidden GIMP 2.6 folder
place it in the script folder, you are done!
Close file manager and terminal.
Open Gimp, new page, select a brush, goto Script-fu in menu bar, select "rotate brush"
fuss with the levers, sorry no preview, changes after you hit OK
Paint, change it, paint, change it, paint
seems to work OK.