When we hung our cornice boards, the L brackets were at the very top and very bottom of the side panels, so it was easy to get a drill bit in there. We used a drill bit extender. DH also has a flexible drill bit extender that can bend but we didn't use that because we didn't need to.
Good luck! We put up 5 cornice boards - including one over our slider - and it was a PITA.
Thank you. That just confirms the way I was going to do it. What's the depth of your cornice board(away from the wall,) 4" or 6"? I also had another idea as well. I could attach the L-brackets first to the wall then hold the cornice board up and screw into the cornice board, then my hand could fit up into the cornice.
Thank again for you help.