I think I understand what you mean. You "can" connect 96W of available power to a 100W inverter, but you will never be able to fully tap what it can do. The only loads you could use would be lights and other light duty devices (led tvs, computer, chargers, etc). Nothing that requires a heavy load start (electric fan, dishwasher, fridge, etc).
Altho, being only 100W, you wouldn't use it on most of those things anyways. 100w is really only for lights or small fans.
So, yes, you could use a buck converter to lower the voltage from 24V to 12V to run the 12V inverter. The biggest hurdle you'll have is the amps required by the inverter. Will it overload the buck converter(s). You need to make sure you have one that can handle the amp loads.