You are much handier than me if you are considering ripping a different piece of wood for a rubrail !! I felt accomplishment from just getting the kit ones on properly. But another type of wood might look nice too.
CLC sells a nice rail padding for the PMD outwales, which I am considering installing at some point. I think you can see it installed on one of their demo PMDs, at their webpage. This might be a simpler solution than trying to measure and cut another piece of wood just right to fit your kit. And in the end, I would think you'd want padding on whatever kind of wood you ended up choosing.
At any rate, I wood get some better advice (read CLC) about the practicality of other types of wood for this application. When we installed our outwales, I was surprised by how much torsion there was on the outwales as they followed the compound curve of the #4 panel. It really took a fair bit of muscle and a ton of clamps to make it stay put in place. A less flexible wood might make things even more tricky.