Bearing complexity & cost efficiency in mind, it may not even be necessary to mount another camera. Simply adding some additional illumination, specifically infrared lighting, could help here. Infrared light would be able to pass through most thin plants, and would be able to pinpoint any unwanted guests on the underside of any leaves. 
I haven’t yet investigated the specs for the current camera on FarmBot, but a majority of cameras use silicon sensors, which are able to see just fine into the near-infrared spectrum. So it is unlikely to be a concern from the camera’s perspective.