In that case I’d add a second check,
for example a PIR pointing into the complete room at ~1,5M hight (depending on the dog’s tail)
or a door sensor unless you tell me the dog can open the door 
You can consider using a capacitance sensor instead of the load cell. It can be set to desired sensitivity and work even with the waterbed.
There’s a rather old project described on mysensors site. Since then there are more stable and sensitive capacitive sensors available such as TI FDC2XXX that allow building of a very accurate bed occupancy sensor.
