I was able to later opt-in to APFS by re-installing High Sierra Public Beta 1 from the original App Store release. Just check the box this time. Since there hasn't been any delta updates yet, the reinstall over an otherwise working setup should be pretty harmless. Mac running with APFS now.
The installation app gets deleted during the first setup, but it's available in Time Machine from Sierra era, if you made one before installing. Or just download it again from App Store.