I've come up with a simpler variant which makes use of the 'dashed-line function. (I didn't use this initially because I couldn't figure out how to get the roundedness of the line segments to match the ordinary stem's and also because I wanted to ensure a full-length dash at top and bottom.)
Hmmm, Setting 'blot-diameter like I did actually affects regular stems and the newly-drawn stems not at all... So I guess the earlier method is the way to go. (That is, unless there is a way to change the rounding of the 'dashed-line stencil, or if such things really don't matter!) Those functions using 'round-filled-box could be tailored to match the capabilities of 'dashed-line.
Best,
David