rectangular callouts

Screenflow's round mouse callouts offer lots of cool options including stroke, feathering, zoom -- but what would be really cool is if one could create rectangular callouts. Workarounds (involving importing PNGS) can be figured out, but what would be great is if the user had a crosshair tool with which he or she could drag out a custom-sized rectangular area callout (with the same options as Screenflow's mouse callout).