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An experienced trans builder recently asked me about designing a circuit to be able to drive an EPC for testing. At first I was leaning towards designing a circuit for him but then I reconsidered and found there may be a much easier way. Found this device that should allow testing of any solenoid and at any PWM frequency and any duty cycle.
I haven't tested it. They claim it can handle up to 8A. Just dial in the frequency and the duty cycle. There also appears to be a connector for a switch to turn the PWM on and off.
If you're testing the EPC in a standalone, not wired in or powered by the harness, this should work fine. My guess is that most likely, it would need an extra pair of resistors and a mosfet to be able to run the solenoid in a running transmission if the normal operation of that solenoid is a ground signal, like in a 4L80E. I can draw that up if anyone is interested.
Standalone EPC line pressure generator 
