Well, it might not be of any use at all but this operator in South Africa used to be very big in agricultural flying and survey.
http://www.avexair.com/
If they don't do this any more they should at least be able to forward you on to someone who does. The most useful man to whom to speak is probably Captain CC, (no relation) an ex SAA skipper of some deserved renown, who still flies 727s while in his seventies.
Best of luck.
cc