Carpet cleaning machines have used electric clutches on their pumps for years, and yes they are reliable. If Russ cannot help you, check with someone like Jon Don, they may be able to assist you.
They are dependable. Aaladin offers clutches on their line of pressure washers. RAHSCO pressure washers also offers clutch models. They are operated through the use of a modified pressure unloader utilizing a pressure switch on the bypass. When you squeeze the trigger the clutch engages. When the trigger is released it dis-engages. Cost for a turn key set up is in the $300.00 range. General Pump offers the clutch. http://generalpump.com/PDFs/100687-Clutch.pdf