The real problem with this theory is that there is no way to guide the Mk-54 on to the pipeline once it hits the water. The Mk-54 uses Active or passive/active Acoustic homing. In active the torpedo can't differentiate the pipeline from the seabed. If you are going to the trouble of attaching a submarine noise generator to the pipeline you may as well place the explosive charge while you are there and cut out the obvious presence of a P-8 in the area.
I understand why you might wish to operate the P-8 at altitude but I'm not sure how accurately you can position Sonobouys dropped from altitude.
FWIW - The pictured plane is from the VP-16 War Eagles based just south of me at NAS Jacksonville, FL.
WWG1WGA
Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)