I am pretty sure that somewhere on the PBP website it says that bag drops will be available!
You ARE RIGHT!
I googled
site:www.paris-brest-paris bag drop
And it is on page 1 of the Plaquette!!! Just to the left of the list of countries "bag drop service to some controls to be designated"
I'm sure you could ask on the French PBP site and see if there is any idea yet which controls are "designated"
The logicists of collecting, moving and storing 6,000 bags is pretty major