The plausible deniability stuff is probably not that useful in court, as you note.
The standard in most jurisdictions is proof beyond reasonable doubt. I don't think the OTR plausible deniability would be considered reasonable doubt, especially if the content of the messages is corroborated by other evidence.
Do you have an example from actual case law? All I have is Wikipedia:
"Plausible deniability is also a legal concept. It refers to lack of evidence proving an allegation. Standards of proof vary in civil and criminal cases. In civil cases, the standard of proof is "preponderance of the evidence" whereas in a criminal matter, the standard is "beyond a reasonable doubt." If an opponent lacks incontrovertible proof (evidence) of their allegation, one can "plausibly deny" the allegation even though it may be true."
In and of itself, a piece of evidence is plausibly deniable. In the face or corroborating evidence, it may not be.
Alas, however, I do not have an example from actual case law.
EDIT: I knew I had seen some case law sort of related to this before and it was in re Boucher and US v. Fricosu. They are not directly related to "Plausible Deniability", rather they compelled defendents to decrypt their hard drives. Certainly in the Boucher case there was reason to believe that the contents of the drive were incriminating (the file names were the giveaway, I believe) so perhaps this does not fit in to the exact facts we are trying to recreate, but those are the only two cases that relate that I can recall.