with regards to the bolt method, I've had luck using electrical tape, a batten, and an open end wrench. I'll tape the wrench solidly to the end of the batten. Then I'll make one wrap of tape around the nut and jam that into the end of the open end wrench. You can also adjust the angle of the nut in the wrench to offset it slightly out of plane if you need to - the tape will hold it pretty much wherever you set it. I slide that batten in until I can see the nut from outside the hole and then carefully start to thread the screw into it. I've also used some liquid nails (a thick glue/sealant) to hold a washer onto the nut. I can get a nut on a screw in the middle of the beam relatively quickly this way.

A rivet washer, however, would be a completely different animal. It would have to be pressed up against the inside of the beam with some pressure in order for the rivet to take it up properly. I might try some hair-brained idea to glue a washer to a chunk of sponge and then tie that to a batten, push that into the beam with the batten and try to wedge it inside the beam to push the washer in place. Use a drill bit to clean out a bit of the sponge through the hole to allow the rivet to penetrate it without moving the washer, then rivet. I've never tried this but I've had crazier things work.

the guys that build "ships in a bottle" might have some neat tricks up their sleeves for this.