This would seem to be the customer's problem, rather than yours, no?
I'm looking at this the same way as tinfoil -- bill them at BTS (kWh only) when the demand (kW) is less than 10kW. Bill them at BTD (kWh & kW demand) when demand exceeds 10kW. (Please define BTS & BTD?).
The "limiters" we've been talking about here are nothing more than circuit breakers -- you (or your customers) could certainly put in a 40A circuit breaker as the service disconnect. That would definitely limit power. But this is a rather crude way to do so -- go over your 10kW threshold and everything shuts off.
You could do something a bit more elegant with a fancier control system -- like a PLC or even a current relay or something -- that would perhaps shut off air conditioning (hmm, not much AC at 40 amps!) or other loads when demand exceeded perhaps 35 amps, and then permitted the AC to start again when demand fell to 30 amps or so. Similarly, you could segregate your loads onto "more critical" and "less important" panelboards, and shut down the "less important" board during periods of high demand. I've never seen such devices used in residential applications, only commercial & industrial systems, but there's no reason you couldn't do this at a reasonable cost.
But still, this would seem to be the customer's problem. Every commercial & industrial location I've ever seen such load shedding installed at, it's always been done at the customer's initiative.
By the way, "peak shaving" is a very similar concept -- if you're doing Google searches on this stuff, you might want to look at that as well.
One more thought, that is probably not economically justified in residential applications, but I'll throw it out there anyway: Energy storage. Use some kind of battery system, or thermal storage, or flywheel, or any sort of energy storage system. Charge the storage system during low-demand periods, and then release/use that energy during periods of high demand to avoid drawing energy from the utility source. This also falls into the "peak shaving" category. The one place where this seems to be *relatively* common in residential applications is the concept of passive solar heating/cooling, putting in a big stone wall or something that stores the "coolness" at night and then "releases that coolness" during the daytime to shave AC costs. Similar ideas might help you achieve your goal as well -- if you could keep your customers drawing at exactly 10kW, 24 hours a day, that's better than if they draw 0kW at night and 20kW during the daytime.
Hope this helps. Very interesting thread.