Well, in concept it seems doable but I am afraid that in practice I could spend countless hours on this and it would not be justified by the amount of work saved.
I have many triac power phase controllers but they use a variable resistor pot. They would have to be modified to use some electrical signal which is no straightforward task.
I have an ac current meter but it would also have to be modified.
Then they would have to be interfaced and connected and, as I said, they are located in different rooms.
It is all conceptually doable but, the more I think about it, I can't see it being very easy and straightforward.
And, even if it did work, I could find myself facing stability problems, oscillations, etc. It could turn into a huge project which, once started, I just have to keep going.
So I am thinking of simplifying to the minimum. I think I can easily make, with a reed switch, a switch that will close when current goes over a set limit. That should be quite simple and I am pleased with this idea.
Then I am thinking of using an RF remote control switch
like this to transmit the signal "current too high". Then the receivers located at each heater, upon the reception of this signal would disconnect for a set number of minutes after which they would re-connect. If current consumption is still too high then the cycle repeats.
This seems to me more attainable in short order.
I guess the question is what happens if the heaters disconnect but consumption is still too high. I guess I could add some beeper as warning.