Why? A lot of EVs these days comes with a heatpump so heating doesn’t use that much power. I live in a cold country where 98% of new cars are EVs and I have never heard anyone asking for something like this. A quick search on popular EV forums shows a few threads from 2010-2014. That’s it. I know webasto has a product like this, but there’s practically zero cars on the road with it installed.
if you get into standstill traffic in the winter, it's actually life-threatening not to have alternative power source that can warm up the passengers (and the battery). One interstate road I'm driving sometimes, has many abandoned Teslas sitting on the side of the road after a sleepless night of staying in traffic during some snow blizzard...
Heat pumps are mostly useless when it is minus -20 C. Diesel heater is the most practical solution. Sodium or LMFP batteries will help, but even with them the range reduction in winter due to the need for heating and increased drag from cold air is just too great.