Jim - No I do not have the tools and don't see any room to attach any type of channel or skirt. Also 2 of the hand rails curve at the end and I do not know how to deal with that.
Frank - No the top rails are not loose, a top dowel will not work, only if it is spring loaded and can go down till inserted and then pop up into the hole. (is that something that exists?)
What if I cut filler pieces in between each baluster and screw them up to the handrail, then brad or even screw horizontally through the baluster into the filler?
I would have to cut each baluster maybe a hair long and bang it in so there is no top gap and a super tight fit to the top (caulk that and then paint). That would leave me with 2 balusters on a curve, maybe just brad those in without fillers, they are next to the newel posts anyway.