If, when you scan your onward BP at the entrance to the lounge and are rejected, then approach the desk and show both BPs (your onward, and your inbound business class BP). You may be admitted on that basis (although, strictly speaking, lounge access is granted only ahead of the sectors for which you are flying in J - and if the lounge is already busy, they will be less likely to grant you access)

This might work in CDG, but I think by the time you arrive in AMS, you're already pushing your luck....

Please report back, and good luck