The older and more popular synonym for ghost farmer is telephone farmer:

"These large-scale farmers include cabinet members and other ministers, members of Parliament, party leaders of the governing Kenya African National Union and senior civil servants. Many are 'telephone farmers' who run things from Nairobi." —Jack Shepard, "Behind Kenyan Discontent," The New York Times, August 11, 1982

Note, too, that I've also heard the term ghost farmer used to describe a person who cultivates marijuana plants on a secluded patch of someone else's land, but I have yet to see this usage in print.