In my previous life as a contracting officer for the Office of Naval Research,
I (we) followed two general principles.  First, just because the person is on a
research appointment, this does not give any greater access to overload
payments than a non-research appointment;  the policy must be consistently
applied.  Second,  the approval to use overload payments was given VERY RARELY
and only in very exceptional circumstances.  For example, if  a faculty member
submitted two proposal for a  60% share  ofappointment and received both
awards, this was not a basis for overload.