There are gear pins that will prevent accidental retraction of gear on the ground or during maintenance. The most common gear pins, however, deactivate the nose wheel hydraulics. If those were active, using a pushback rod would get rather interesting. Would the rod snap and go all over the place breaking handlers? When connecting or disconnecting the rod a pilot stretching his legs could make it flail about too, again breaking handlers. I would not have been amused if that had happened while I was working there.
Cheers,
/ft