ok, i will nibble ...
there is/was a site called
http://lbwindsurfing.com that was dedicated to longboard windsurfing
however, i say is/was because the site sometimes disappears or like currently lies VERY dormant
there are NO forum entries from 2016 and some of the latest 2015 entries are from me ...
getting back to starting point or getting through wind shadows and zero wind issues are not just affecting longboard users
obviously pumping has to be part of the equation
personally find that even 2 knot winds are so much better than trying to pump in zero winds
if one has zero wind and there are also waves and/or current to contend with

rather than explaining the pumping, will put a link ...
howtowindsurf101.com/windsurfing-pumping/ &
howtowindsurf101.com/how-to-get-back-if-the-wind-drops/if I am heading back and have wind / equipment or fatigue issues, I will swim to shore (if less than 1 km)
this is not so much an option when going out, but i have seen videos in Hawaii where they do just that ...
sometimes i wonder if we don't need a really tiny waterproof electric motor at the back to just gently urge the board forward in no wind situations