Something super basic.
the only folks who'd have to be involved are those who call holding... the crew chief is typically the Ref...ump... I can never keep those straight...
Moreover, it doesn't have to be as blatant as "hey Ref, we're fixing this game"
All it needs to be is some really basic memo sent to the crew chief "as per the League outlines and a review of prior games, the NFL wants to emphasize that holding be called even if it happens away from the ball" or conversely, "as per the League outlines and a review of prior games, the NFL wants to ensure the flow of the game and emphasize those calls near the ball or that specifically affect the outcome of the play."
Pretty innocuous, but it sends the message.
Doesn't require all the refs to know, doesn't require some grand scheme... just a few people who know and communicating through open channels. Innocuous language can have secondary meanings that have been established in advance.
Doesn't require a "grand conspiracy"
Doesn't require a ton of people in the know
Doesn't require elaborate schema or spy level codes or secret communications
The NFL alters its "points of emphasis" regarding calls all the time. But the whole point is that the calls are meant to be called equally, regardless of the team.
The key point is deciding which team benefits unfairly over another team.
It's a big difference between "make more holding calls" and "make more holding calls against Team X."
You mentioned "secondary meanings that have been established in advance." That's the key part to make it plausible. Who establishes these secondary meanings, and with whom are these meanings explained?
I recall now this discussion from a few years ago, so at least I remember where you're coming from. Sounds like you think there's a lot of "wink and a nod" pressure on certain refs to call things certain ways.
All I could say is, the more overt and blatant the pressure, the more effective the "steering." But if the pressure is subtle, the steering would be less effective.
Cheers