I'm willing to bet oxytocin would work. One effect is to make people more trusting, so if you coupled that with an authority figure in a white lab coat intoning "Take this pill, and you'll get better," it would probably have a high placebo effect rate.
My brain hurts trying to imagine how this drug would undergo a DBRCT.
Easy. You set up a double-blind experiment in which the constituents of a random sample of the population are randomly assigned to one of two groups: experimental group or control group. You administer the experimental group the new drug (dammit, now I've got Huey Lewis and the News stuck in my head...), while the control group gets a placebo. You then administer both groups further placebos, and measure their response to those placebos.