I've been involved with helping a few folks Ray. I know it will depend on the trike model, but I know some folks that are using the front brake lever to drive the rear and front calipers. Disconnecting one front caliper on an Ultra framed trike with dual calipers will work to stop the trike buy clamping three calipers...one front and two rear. It will depend on the brake caliper and MC volume. There are also some "very expensive" handlebar controls that will drive both front and rears, but the cost is up there around $1700 or so.
Some kits use just one caliper in the center of the rear axle, so I have no doubt that the stock HD brake MC would drive the one rear and one of the fronts. I think the customer will be able to pull the brake caliper with just two fingers.