There doesn't seem to be a whole lot on the net, but I found a 1912 Case Steam Tractor rated at 75 hp and weighing 24,000 obs empty. That's on the order of 0.005 KW/kg, dry.
I found another, a Burrel from 1903 with a nominal rating of 8HP, weighing 11,400 kg loaded. That's around 0.0005 KW/kg,
but nominal horsepower had little relationship to actual output by then, which looks like it was about 5 times nominal for that class of engine, so say 0.0025 KW/kg.
http://www.paxmanhistory.org.uk/paxmanTE.htm suggests about the same.
They were not weight efficient, that's for sure.