Upon some reflection, I think one can say this: first, let's say the protein in question is 30kD, with a solvent content of 50%, and we know that solid protein density is ~1200mg/mL. Therefore, the protein concentration in the crystal would be ~20mM. Because Kd's assume infinitesimal ligand concentration, I think that neglecting ligand depletion effects mentioned by Edward Berry, say by having a huge reservoir or transferring the crystal to an appropriate soaking environment, that all ligands which bind with a better than ~20mM Kd should be bound in that crystal, even at extremely low ligand concentrations, so changing [ligand] from 1pM to 10mM should not change occupancy much, again assuming equilibrium and neglecting ligand depletion.
JPK
