Decided to go ahead and paint behind the stairs with the DryLok Paint. This really wasn't an easy decision for me, since eventually I'm tearing out the stairs and putting in new ones. Not that it's a huge deal, just that I debated leaving it until the stairs were out.
The determining factor was that I have no urgent need to tear the stairs out, so it just makes sense to paint this section the best I can and fix any missed parts later. Loving this stuff, it's thick, goes up so easily and looks great when done. Yeah I'm skimping on this part of the project by not going all the way up the wall. That's because I'll be putting a wall over this and won't see the paint anyway, it's just for water sealing in my project.