It is very simple. These passengers are due to board last after everyone else is on. They can be checked at the gate to make sure they only have small items that will fit under the seat. Then onboard the crew will have already shut all the lockers and can monitor they are not reopened. There is unlikely to be much space left in any case.
OTP is expected to be much the same, as although these passengers board later there will be less "fights" for overhead locker space. I believe these economy basic fares will be for middle seats only but that may be incorrect