I’ve been looking for a similar feature as well (see this post and this post). The RW themes that feature vertical sidebar navigation have limited options in this regard. The various frameworks (Foundation, Pure, Responsive FreeStacks, and Bootsnap) may offer a few more options but sidebar navigation options overall seem under developed. It is possible that one of the menu stacks (i.e., my menu by Nimblest or defligra’s SidePanel CSS Stack) may offer this functionality but I don’t have these stack so I can’t say for certain. If you find a solution please post back.
Cheers,
Doug