Eu escrevi um script. Parece fazer o que você pede:
#!/usr/bin/env zsh
# Custom suspend
#
# (That 'zsh' up there can be switched to 'bash', or
# pretty much any shell - this doesn't do anything too fancy.)
#
# Dependencies are mostly xfce stuff:
#
# xbacklight
# xflock4
# xfce4-session-logout
# Set how dim we want the screen to go (percentage, out of 100)
dim=5
# Pack up your toys
previous_dimness=$(xbacklight -get)
# Turn down the lights
xbacklight -set $dim
# Lock the door (this requires a password to get back in)
xflock4
# And go to sleep
xfce4-session-logout --suspend
# When we wake up, turn the lights back on
xbacklight -set $previous_dimness