The CTs have leads about 5 feet long. The leads can be extended with simple headphone extensions..With two extensions you can measure mains with 400A CTs at the transfer switch. That would be the more economical way to do it.
That works too. Downside is that requires four inputs on the IotaWatt. You combine them by defining an output in the IotaWatt that adds them together. The upside is you would know how much power is being used in each panel.