As a former fed inspector I can simply offer this: if you obey your FOM and the regulations you won't get in trouble. If I'm sitting in the back or the jumpseat, on official duty, and I hear an FO or Captain rambling on while taxiing, I cannot simply "let it go" because I would be derelict in my duties as an inspector. What I and 99.99% of us would do would be to pop our heads in on the way off the airplane, say "nice flight, thanks for the ride, and maybe just keep the PA chatter in mind.." or something like that. Contrary what you think, Feds are not out to get you. But we do have a job to do as do you.
And no Fed jumpseating is ever "just looking for a ride home"...each has a very specific checklist to follow so stay on your game when a fed's on board (and when there's no fed on board).