I don't have any of the answers, only more questions.  Why only male clones?
Are they more productive of biomass than females?  I have not noticed that
male willows are bigger than females.   Willow pollen can be very important
for spring build up in honey bee colonies.  So can the nectar.  If the
objective is to maximise the energy that can be extracted from an area of
plantation my guess is that a mixture is best.  Probably the optimum mix will
be different from season to season depending on weather patterns.  The bees
like both pollen and nectar and maybe the willows will like a few ladies
around as well.
Chris Slade