There's actually quite a market already for controller boards to allow people to use Tesla drive units (inverter/motor/differential setup) in non-Tesla vehicles. Not sure if such an approach would work if there are significant electrical differences between driving a motor and driving a utility backup power generator, but it's at least possible that it could work. Though on the other hand, for now Tesla drive units are in demand for vehicle conversions, so it would be kind of a waste to take the inverter out and to discard the motor and gearbox (which aren't useful without the inverter).

You're probably right that just engineering a specific solution or ordering them from a supplier that makes such things is probably less hassle and possibly cheaper in the long run.

I'm currently doing an EV conversion. I'm amazed that you can buy a box that's about ten or fifteen pounds or so for a couple thousand dollars that can handle about 90 kilowatts of power (in short bursts), converting around 100-200 volts to AC power to drive a motor. That's probably enough to power a whole residential block by itself, and yet it's not even a very powerful controller compared to what you find in a high-end OEM EV. I'd expect anyone building a large scale grid backup battery storage system to have access to inverter parts at least that good and at least that cheap, and if they don't they could afford to design it themselves.