My only concern would be whether or not the oil pump will pump oil with it spinning backwards.  If it wasn't spinning crazy fast probably not a concern either.  A lot of marine engines spin in reverse, and have heard of several older engines (detroits maybe?) That would sometimes backfire and start backwards or something along those lines.  I guess only concern would be sucking some dirt/trash in through the unfiltered exhaust side.  Probably not an issue where you were using it and that short a duration.  Others will chime in with real/firsthand knowledge.