Keep them indoors at 18-6 and put 'em outside anytime and it'd be just like an indoor flip. I keep mine at 15-9 in a greenhouse (just standard 9 Watt LED's) so as not to garner too much attention but anytime I want to flip a vegging plant I just move 'em outside. I've done it in winter here.
Tomorrow, on June 20th our daylight will be 14 hours and 18 minutes, sumer solstice, longest day of the year for those of us above the equator, so as long as I keep enough extra light they'll continue to vegg and I can flip them whenever I want. You'll no doubt be seeing and reading about it. I can stagger out harvests and breed on my timetable