How strange, I actually seem to have got a macro working now for this!
I haven't given it extensive testing, but give it a try, it seems to work every time here so far.
https://stash.reaper.fm/oldsb/174856/...ected-Area.rar
Stereo Render of Selected Area = Ctrl-Shift-R
Mono Render of Selected Area = Ctrl-Shift-Alt-R
I hope it works for you! Modify as you want (NB: do NOT consolidate undo points, or it will screw right up),
I threw glue in because it wouldn't work properly for more than 1 item on the same track, and anything could happen on other user's systems when using undo trickery...but so far here, so good!