Having read the regulations about the Lockdown League and the referenced Online KO/Plate, there is no reference to "the system announcement is made as private to the opponents". These seem to be the closest things to it (from the Online KO doc):
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17 Teams should exchange system cards with each other before the start of play, either in advance by email, or by having completed online system cards available for their opponents to refer to before the start of the match.
18 Normal alerting and announcing rules apply, subject to two exceptions:
a If the software allows, players should alert their own calls, and not their partners, ensuring that only their opponents and not their partners can see the alerts and any explanations.
General explanation of system is a normal thing, but it's not "their own call", so that doesn't apply (given Cyberyeti's acknowledgement that IRL, this announcement is not always remembered to be done before cards are in hands, I think, in fact, everything went "normally" per pp.18).
What does, 100%, apply, is the following from the Lockdown League doc:
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This is a friendly (competitive fun) league
Yes, I elided the rest of the sentence explaining that it has to be regulated. That doesn't change the fact that even attempting this is a clear violation of the intent of the league and should be treated as such.
Having said *that*, I stand by my original comment: given that it could be unclear, and given that BBO makes it so that cards are seen as soon as all four players are seated, I would call up Charlie or Gordon as requested and confirm that this is a valid method, required by regulation, and therefore is subject to the carveout in 40B2d.
The end of that paragraph (past my elision) says:
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I will stress that to request a ruling, you must send [gordontd] your request within 20 minutes of the end of the match.
I assume that that was done perfectly, and that gordontd's response was "I am roundfiling this, and will pretend it was pitched unread. That means I have not taken note of your attitude and unwillingness to play this league in the spirit it was obviously, even to you, intended, officially. Which means that your team is not disqualified - yet." or a less polite version of the same.
When I go to sea, don't fear for me, Fear For The Storm -- Birdie and the Swansong (tSCoSI)