How to add a secondary circuit breaker, limited to cutting a single appliance
You can achieve this with 'home automation'.

What you're trying to do is called 'load shedding'.  It's not a common task in a residential setting, but it can be accomplished with home automation technologies.

There are z-wave devices to measure current with an amp clamp, and to control appliances in a split-phase (biphase) set.

You would get one amp-clamp and instal it in the panel, measuring current accross your feeder wires.

Then you install one of those relays at each appliance that you want to be able to shed.

You also will need to purchase or build a controller box that talks to both the amp clamp and the relays.  MiCasaVerde is one choice I've worked with personally and I bet it could do what you want.

Then the fun part.  You get to write a program that runs on the head-end to constantly measure amperage draw and controlls the relays accordingly.

Be careful that you don't create some loop that rapidly toggles the relays though.  At high amperages, I could see that being a nuisance to adjacent units (power blinks) or worse case you could overheat and set one of the relays on fire from the arcs that switching causes.

Good Luck!  And remember to share your code and parts list here once you've got this working.