After understanding how the "set debian hw keys" script works, I've edited the /home/user/.xbindkeysrc.scm file to add these lines :
; Fullscreen key (Left)
(xbindkey-function 'Left
(lambda ()
(run-command "xmodmap -e 'pointer = 1 2 3'")
(reset-first-binding)))
; Fullscreen key (Right)
(xbindkey-function 'Right
(lambda ()
(run-command "xmodmap -e 'pointer = 3 2 1'")
(reset-first-binding)))

Now I can toggle right-click and left-click with minus+right and minus+left