Is there a role for checkpoint inhibition in patients with advanced, high PD-L1 expressing, chemotherapy-refractory thymoma?  

No, these patients should not receive PD1/PD-L1 checkpoint inhibitors (CIs) outside the context of a clinical trial. CIs such as pembrolizumab carry activity in chemorefractory thymomas, but also impose a very high risk of severe (grades 3/4) immune-mediated toxicities in this population, regardless...

I agree with @Bernardo H. Goulart. For now, I would not treat thymoma patients with immune checkpoint inhibitors outside of a clinical study, based on the serious and frequent immune-related adverse events seen in this population as detailed above. There are several single agent chemotherapy options...