Thank you! it's actually a clearflag, not a setflag, but it works! Though you also have to copy the previous setvar.
So it looks like this now:
setvar 0x4071 0x1 (makes the scripts associated with the event run)
clearflag 0x62 (this makes Bill's sprites appear on Cinnabar Island and in the PC in Cinnabar)
...and then the rest of whatever you want.