I am familiar with two 135/91 companies and its is covered in both their ops manuals. Both have the same "answer." A Part 91 return is allowed but it must be requested by the pilots and then approved by the chief pilot. If anyone says they're too tired, an overnight is the answer. An overnight is planned from the beginning so if the pilots are too tired then they just go to the hotel without making any calls. To me it seems to be the right answer.

C9