Except in this case, runsvdir executes runsv, so you have to tell runsvdir to chain-load.
You want runsvdir to be your reaper, so you'd just run "local-reaper runsvdir scandir" instead of "runsvdir scandir". If you wanted runsv to be your reaper instead, it would indeed be somewhat more complicated (you're need to wrap runsv), but this is not what the OP asked for. -- Laurent