As it turns out, it's not the actual painting/priming that is difficult one-handed, it's the prep work. Perhaps this should not be shocking, but I was a little surprised... But after sanding the last coat of compound, then cleaning up the dust, then washing the walls - that was one tired right arm!
At the end of the day, the long hallway wall is about 3/4 primed. I had some small technical difficulties along the way. Like - R2 kindly set up the long ladder for me to reach over the stairs before she left for the day, but I needed to move it in order to reach one spot on the wall. Um - can't move the long ladder using only one arm. So - that spot not sanded, therefore not primed. And the wall over the stairs would require lots of ladder shifting to paint, so that one isn't done either. I'm also messier when painting using only the right arm - but I have to say, no longer looking at the red/yellow wall is a vast improvement. Even if I'm now looking at splotchy primer ;) Look for more lame-handed painting to come!