So, after all of this speculation I thought I'd just ask at the source and dropped an email to AF's German helpdesk, asking them about the baggage limits for me as a PM on a AF operated, DL coded flight and flying on DL ticket stock. Here is the somewhat confusing answer (translation below the original text):
Geehrter cygnus
vielen Dank für Ihre E-mail Anfrage.
Als Platinum Vielflieger haben Sie bei Air France mit einem Air France Ticket
weiterhin die gleichen Gepäckbestimmungen. Falls Ihr Ticket von Delta
ausgestellt wurde, gelten jedoch die Delta Bestimmungen.
Wir möchten Sie bitten, dass Sie sich bezüglich weiterer Fragen telefonisch mit unserer Reservierungszentrale unter folgender Telefonnummer in Verbindung setzen:
Deutschland 01805 830 830 (0,14EUR/min)
Sie erreichen unsere Kollegen zwischen 08h00 und 20h00 Montag bis Freitag und
09h00 und 17h30 am Samstag.
and the translation of this is the following
Dear Cygnus,
thank you very much for your inquiry.
As a platinum frequent flier on AF with an AF ticket the same baggage regulations continue to apply. If your ticket was issued by Delta, however, the Delta regulations apply.
We would like to ask you to contact our reservations center under the following telephone number...
If you wonder about the confusing sentence (As a platinum...), yes, it is that confusing in German as well - I have no clue what they mean with "the same baggage regulations continue to apply". The important and even more confusing part, however, is the one boldfaced. Not that I believe it to be true given DL's international conditions of carriage quoted further up in this thread, but, well it is an additional data point and I'll print out that email and take it with me on my next flight...
I'd think, however, that we should try and get some more information out of AF, so I suggest that somebody else asks a similar question to AF (in a different country). Let's see what they say!