I started by finishing the last few rivets on the inboard upper skin. The remaining rivets were just the ones along the main spar flange, so I decided to try to backrivet them by flipping the wing with the skin on the work table. I had to use some folded cardboard to slightly raise the angle of the wing so that the flange would rest solidly against the backriveting plate on the table. Some blue painters tape held in the 20 or so rivets while it was in this position during the riveting. It surprisingly worked quite well and was very quick.
Since I had removed the outboard upper skin in the previous session, this provided a good last opportunity to replace the bolts and add safety wire on the other remaining aileron bellcrank bracket. The safety wire pliers that I bought from Harbor Freight are surprisingly functional, at least relative to some of the beat-up pairs that I've used at my mechanic's shop.
Finally, I clecoed the outboard upper skin back into place and started riveting the easier rivets along the rear spar flange. I got about half-way done with those before deciding to wrap up for the night.