1) Instead of using a 555 timer, which is based on a fairly unstable R-C time constant, use a digital counter. Count cycles on your supply line, and then all of your signal generators and detectors will have the exact same time base.
2) Consider using a nice machine detectable toggling pattern on the load, something that a digital ammeter with appropriate software could very easily detect. For example, there are standard binary digit sequences that are well known for use in spread spectrum communications systems. Different binary sequences can be easily distinguished. By turning your load on and off at for individual cycles of your AC supply, following one of these digit sequences, you would get a very well defined load pattern that was easily distinguished from other load patterns. Turning the load on and off at this rate could easily be detected and differentiated from other loads.
This toggling pattern could easily be implemented in using a cheap ($1 USD retail) microcontroller chip, driving the sort of SCR used in a light dimmer.
3) The above machine detectable toggling pattern could easily be combined with a human detectable one. Rather than flipping between 'on' and 'off', flip between switching full cycles on or switching only half cycles on, so that the human readable load changes by a factor of 2 and the machine readable load is always available.
-Jon
