I understand what you want to do, and it sounds like it shouldn't be too difficult.
One method that I was thinking of, was to use an AC current sensor connected to my heating system.
When my heating system was using a lot of power, the current sensor would trigger
a GTI (Grid Tied Inverter) to come on and pull power out of my battery bank.
But, the GTI would be limited, so it would never make more power than I needed for my heating system..
When the heating system power use dropped, the GTI would disconnect.
My DIY circuit breaker design could be hacked to do the switching.
http://ecorenovator.org/forum/applia...r-limiter.html