Close, but it's actually run with two large groups: one that gets the real shot, and one that gets a fake shot. You then compare the two groups, rather than comparing with the population.
But paying people to be studied tends to be effective.
But I don't think the particular aspect of "making sure you have enough participants" interacts very much with ethics. Especially when the failure state in question is an honest failure to attract them.
In that case, someone at high risk for HIV infection could be prevented from getting treatment because of the study protocol. The ethical solution, which the authors of the study chose, was to provide the highest standard of care as the placebo - in this case it was Truvada.
Since the treatment was actually even more effective than the existing standard of care, it was a home run success. Their conclusion is even stronger than it would be if they used a non pharmaceutical placebo, so in retrospect their decision was clearly correct.
In this case the study cites that as being unethical due to the high prevalence of HIV in the target population. So the actual trial gave some people the shot, and some other people the known-working daily pills as a control.