In reply to the question in OP (again based on the position in England & Wales) but it is, AFAIK, a fairly general concept.
'Privilege' - i.e. the absolute confidentiality of certain material and the fact that there can be no enforced disclosure in legal proceedings, applies to specified material, commonly 'legal privilege' (discussion between lawyer and client), medical records, and the confession of a penitent to clergy (note previous discussions on the latter - if the clergy report the confession it loses its confidentiality).
One point sometimes missed is that in all 'privilege' cases the 'privilege' belongs to the individual, not to the other party.
So, if the individual willingly discloses the content of the legal discussions, or his medical records, or his confession, etc., the other party has no claim whatsoever to enforce confidentiality - the privilege is not his to claim.
This, it seems to me, would apply to JCs. If the 'subject' (or 'poor shmuck', in the vernacular) who is the only one who could legally claim the privilege of confidentiality chooses to waive that, no-one else can insist on it.
Just my thoughts.