Yes, that's something like what I'm doing. The ends of the 3" balusters, though, narrow to a 1-1/2" tenon.
I tried to design a trim piece, but it would have required a large 4-by piece that I couldn't find at the local Home Depot -- which is the only option in this rural area.
I'm back to the option of cutting the extra balusters into 4.5" sections, and joining those to the back end of each tread to make for a longer baluster. The 35" baluster would go on top of this platform.
I'm not sure how to join these pieces securely. I bought some 3" all-thread lag screws, thinking I could use these, along with wood glue, to secure the 4-1/2" platforms to the treads. For the balusters with tenons, I was going to drill out mortises for the tenons, and also use the double-ended lag screws and glue to hold them in the treads or the platforms, as the case may be.
Does that sound about right, or is there a better way?