No promises ... but if we need to duplicate chords notes how about this:
The command would be
Chord dupnotes 1:-1,-2;5:-1,3 <bar2> <bar3>
which means that the root (1) would be duped to an octaves 1 and 2 below, the fifth one below and 3 above.
In other words we have Pitch:Octave[,octave][;Pitch:octave..] for each bar in the seq.
Volumes (velocity) for all the notes would be set using HarmonyVolume.
Very ugly, and even I would be reaching for the manual each time I used the command
Honestly, I'm not convinced that this is the best way to go. If you use a separate track one neat thing you can do is to separate the duplicated notes and the main chords into different tracks (even if they share a midi channel). Then, you can do mangling on the different tracks in an editor, etc.
I'm still suggesting that if you need to fill out a chord that you use a separate chord or bass track.
The only problem I see with using separate tracks is if you have a lot of random stuff happening. It might make sense to have a facility to connect tracks together to avoid this, but I'm not convinced that it's a problem.
Duproot as a very early command and I probably wouldn't do it that way today. Don't know for sure.
Before I do much more on this, I'd love some comments.