Actually, CryNGRoad had a very clever solution to this issue:
I would suggest just one more addition to that command in order to completely duplicate the actions taken by nautilus-share on Gnome for a guest accessible share:
Just create a custom action (Edit menu>Configure custom actions) to run the following command only on directories:
net usershare add %n %f "" Everyone:R guest_ok=y
This way there is no need for Nautilus, Dolphin, or any other aquatic animal.
net usershare add %n %f "" Everyone:F guest_ok=y && chmod 777 %f