As other have said, it looks like your best bet is to tap the 100A feeder conductors that land on the top service disconnect in the panel. It those feeder conductors don't have an OCPD on their load end, you will need to install that. If the conductors you use to make the interconnection are rated at 100A, I don't believe that is technically considered a tap, and so you wouldn't have to follow the tap rules. Just run the circuit into a 6-space 100A MLO load center and land your inverter circuits in there. Of course you will have to meet the Rapid shutdown rules and utility requirement, which may dictate additional disconnects.