Can I insulate flat roof using this method?
Certainly seems like it should be some improvement over the traditional method - how much is not clear, since the bags will presumably pack down and lose most of the insulating air spaces under soil pressure, but even "more or less solid" LDPE would be a slight thermal improvement over soil, if you have access to a lot of cement bags.

A rigid foam insulation (or foamcrete) would be more effective in this application, but presumably expensive or hard to source locally.