ard-reset-arduino resetting Leonardo/Micro via USB works on OSX, fails on Ubuntu
I'm somewhat embarrassed as to the source of the problem.  It wasn't an OSX vs Ubuntu issue.

Rather, on the Ubuntu system, there was an another application that was holding the USB port open, so attempts to reset the Leonardo were failing.

Rather than delete this post out of shame, I'm leaving it here in case someone else stumbles across the same problem.