I would recomend you to add one more MCU which can controll all important stuf.
BTW I intend to do exactly what I said. One more MCU which will controll all analog inputs of 16F876A, also it will show all those values on the LCD (or GLCD) and will be connected over RS485, as a slave, to the master, which controls also some other things.
I think this is easyest solution, becouse you don't need to study inverter code. You just control few important points of the inverter.
But everything depens on you, how much time you decide to spend on this project, a level of your knowledge is very important and so on ...
Regards