Though not as convenient as the camera or volume buttons, the lock slider between the right speaker and audio port can do the trick. dbus-monitor --system shows several messages when I press it, but I think the most useful one out of them is this one:

signal sender=:1.13 -> dest=(null destination) serial=7006 path=/org/freedesktop/Hal/devices/platform_kb_lock; interface=org.freedesktop.Hal.Device; member=Condition
string "ButtonPressed"
string "cover"