nojh
to go way back to your original question - yes - I have seen an engine contaminated by swarf, though not from the tanks, but from a failed engine driven fuel pump.
And just to put peoples terminology into perspective shouldn't we be referring to this material as DOD not FOD. Since you are suggesting the material was from inside the tanks. If someone had taken something from outside the tank, left it in there - that would be FOD. DOD - Domestic Object Damage, i.e. from inside the system.
Further to your questions, would you be better asking what procedures are in place to replace engine LP fuel filters at a set period after fuel tank repairs. I thought this was common-practice, if not in-fact an AMM requirement?
Brgd's
N1 Vibes