All translators for daytime AMs are allowed to operate 24/7 if they wish to.In at least one case, an AM daytimer is allowed to operate and program their FM translator(s) 24/7, since the FMs are allowed to operate 24/7 with the (controlling) AM on during only during the day, IMHO an AM station should be allowed to operate only during profitable hours (reading RD.com, it seems that time is in the morning).
Otherwise, the FCC will not allow an allocation to under utilize its frequency. So running just a few hours a day is not allowed.
Operating limited hours will not allow other stations to operate. It is of no benefit to the community and deprives them of a greater schedule. Nobody is going to spend the money to get rulemaking proposed for this as nobody wants it.It would likely require a rules change by the FCC, but, AFAIK, the FCC cares about keeping interference low by specifying technical parameters, if an AM station wants to be OTA for ~2hrs/day, I'd say let them.