fuel ballast
MD FAA manual says: " Auxiliar tanks may be used for ballast fuel only on airplanes with the fuel fill line drain valves deleted per Service Bulletin 28-51, or the production equivalent"
I know that the swiss company Balair did use these tanks for balancing porpouses.
1.000 lbs of fuel at FWD AUX creates - 6 IU, while at the AFT AUX tank just + 2.5 IU.
My company doesn´t allow this procedure and max unbalance betwen AUX tank is 400 lbs.