Several hours later and I manage to find out the extended commands for the LW11G dimmer unit. Can't find these anywhere else mentioned on the web, so for future generations:

# Extended X10 control of LW11G dimmer
#
# Unlike other L*11* modules the LW11G
# seems to only respond to code 53.  Set the data to
#
# 0 = immediate off
# 255 = immediate on
# 1-254 = slowly dim or bright to that level, turns on if
not already