I suspect in most cases, you'd be able to recycle control data from a previous study, and hope their placebo effect should be similar enough; or that your treatment effect is substantially stronger than optimism.

But given the kind of conditions we'd be likely to use this methodology for, I don't know if the placebo effect has a strong effect on outcomes.

Basically, you make a fake control group using data from previous patients: eg. 90 days into a clinical trial, you'd expect X% of patients in the control group to have died, and if you know that for certain, you don't need to have a control group actually sit there and die for you to confirm it.