Regarding the use of steady or flashing rear lights for Randonneur riding, the requirement is indeed a "steady" rear light. It is my understanding that this is for the comfort of following cyclist's. As you can imagine, there is lots of group riding in PBP, and lots of very fatiqued riders. The feeling is that the flashing rear light will be mesmerizing, and lead to additional rider fatique and possible riding errors.
Regards,
Larry