What I recently did was to use my BAEC number during booking, then change it to my other FFP number on finnair.com to select seats.

After check in and accessing the lounge, I went online again and changed it back to my BAEC number on finnair.com, then reprinted the boarding passes/reloaded them on my phone, and used those new passes at the gate.

Worked without problems!

(I find it more reliable to try doing things yourself rather than relying on lounge agents, who may otherwise refuse your request or what not.)