I would agree that it's not entirely obvious in the MPSH case up-front, since there is no population information available.
However, unlike for molecular spectra, the MPSH energies are shifted such that E=0 is the (average) Fermi level of the system, and thus by definition the HOMO has the highest negative energy and the LUMO the lowest positive energy.
It doesn't really make sense to talk about populations of these levels, other than in the sense of applying the Fermi distribution, but that is precisely equivalent to the discussion just above
At finite bias things get a bit more complicated. It is however important to be aware of the fact the strictly speaking these are
not "real" HOMO/LUMO levels. They are some kind of projected levels that
correspond to the original molecular levels of the isolated molecule.
