For my two pen'orth, I have a dove tap fitted to my AJS, it works a treat and I stop the engine by switching off the tap just to make sure it's working OK.
I'm not bothered about the theoretical draining of the feed pipe, I'm sure that by the time I have switched the Dove tap on, opened the fuel tap, tickled the carb, adjusted the advance and air lever's, said a prayer or two, put my helmet and gloves on and kicked it over, that oil would have re-filled the line anyway.
I think this is one of those situations where you can get a bit paranoid and over think the situation and imagine problems that probably dont exist.