I am almost completely certain that you CAN insert an inductor in the transformer primary and accomplish the same form of inductive regulation as with an inductor (or resistor ) in the secondary. Using a variac as a variable inductor is okay, provided it has the required current capability. The series inductance (and thus the reactance)of the variac will vary approximately as the turns(voltage) ratio squared, making it quite nonlinear.
If you look at the equivalent circuit of a transformer, it seems clear that the series inductance for output current limiting can be on either primary or secondary side, as is most convenient. The reflected impedance on the primary side will be the secondary impedance divided by the transformer's turns ratio squared.
Dave Cooper
Created on Monday, March 26, 2001 2:18 AM EDT by David Cooper