If you were on one ticket, BA should be able to issue you the BP no problem at CDG. On two tickets (especially at an outstation not LON), you may be in for a bit of a fight. Doesn't hurt to ask though.

CDG is not a BA or oneworld lounge (sadly), but a rather crappy AF one which BA pay to use. There is no oneworld access for bona fide card holders (e.g. AA ExPlat on BA that day), let alone that onward access rule, sorry.