You can do randomised trials which are unblinded. Several have been done, including community studies in Denmark, Bangladesh and Burkina Faso. Mask adherence was very low in the intervention arms in the latter 2 of settings, which were done as cluster RCTs. In Denmark it was an individual level RCT so cannot assess masks as source control.

In short, inability to blind brings potential biases but is not a reason to not perform RCTs.