Thank you kindly, Eric. So, it looks like the following does the job. (The "puts" lines are just for informational output. The real work gets done by the "setConstruction" lines, of course.

plenumSurfaces = plenum.surfaces
plenumSurfaces.each do |plenumSurface|
  if plenumSurface.surfaceType == "Floor" and plenumSurface.outsideBoundaryCondition == "Surface"
    puts plenumSurface
    puts plenumSurface.name.to_s
    puts plenumSurface.handle
    plenumSurface.setConstruction(construction)

    puts plenumSurface.adjacentSurface.get
    puts plenumSurface.adjacentSurface.get.name.to_s
    puts plenumSurface.adjacentSurface.get.handle
    plenumSurface.adjacentSurface.get.setConstruction(construction)
  end
end

I agree about checking for adjacent surfaces, but in my (limited) case, I know I have those. Cool - another problem solved!