The link provided is for launching your boat but if you could use a similar set up with the line to the winch post to hold the boat at the point you are holding it with the engine then you wouldn't have to leave the boat while its in gear. You could reach over the bow and attach the line and then take the boat out of gear and shut down and boat would not roll off the trailer as it would only roll back till any slack was out of the line. Then you could get out of the boat and winch to the bow stop.
http://newenglandboating.com/fishing...-yourself.html
Mike