I don't want to give up too much space on the stairs themselves, and I don't want to try to drill holes into the tops of all those treads. There's just too much potential to screw it up. So I'm going to attach the balusters to the outside edge, and I'm just making plain rectangular ones from 3/4 inch plywood. Will they flex too much? I hope not, but I won't know till they're in place. For the railing, I'm using a basic round railing, but I ran a dado the length of it to receive the balusters. I made it the depth of the plywood partly for the extra surface area (better glue adhesion) and also so I can use plywood strips to fill the spaces between the balusters.
I'm addressing my flex worries in a couple of different ways. The newel post will be attached to the bottom step with two lag screws, and screwed to the second step for good measure. And in the picture to the right, you can see where I've notched the top of the railing to attach it to the beam that runs along the edge of the staircase.
More tomorrow. I'm taking the rest of the night off. Between cleaning the workshop to get ready for this project and doing the work I've done so far, I'm beat. To quote an old Le Guin book, "Tired is stupid." If I try to do more tonight, I'm liable to screw something up.