"Getting a second useful job out of the most expensive part of an EV makes sense. "
No it doesn't. First, it's not a "second useful job" very often, if ever, and not for many buyers, plus it decreases the lifetime of the "most expensive part of an EV". It's an option, it doesn't make sense for many people, perhaps any when you consider the long term cost.
"a reasonably priced way to keep the fridge and lights running is a pretty appealing extra..."
For many that's a rare problem. For others there are alternative solutions that don't wear your vehicle out.
"Making the hardware affordable is a good start; the electricity savings still have to survive the subscription bill."
For the subset that could receive any benefit at all. I get no value out of such management, my electric is flat rate. Plus I don't have a service interruption even once a year.
Just because it can be done doesn't mean it's "useful".