What are the legal ramifications if one was really Satoshi, denied it publicly, but then later proved to really be him?
In general, the person's false denial is irrelevant for legal purposes. One would have to add very special circumstances for the person's misrepresentation to have any legal ramifications.
Those additional circumstances would have to be material (1) for claims where the intent and/or effects of the falsehood meet one or more of the prima facie elements of the claim(s) at issue (examples: fraud, defamation, tortious interference with business/relation, etc.), and/or (2) as a hindrance to fact-finding in a case (examples: perjury, spoliation of evidence).
Furthermore, the circumstances that would render the false denial actionable are most likely very intricate. For instance, two of the prima facie elements of fraud are [plaintiff's] reasonable reliance and resulting losses. It would be very hard for a plaintiff to prove that (1) he reasonably relied on the person's public denial on a dubious matter with seemingly no financial or criminal repercussions, and (2) in a way that led the plaintiff to losses.
Not only the matter of Satoshi Nakamoto's true identity is/was an elusive matter (thus defeating an argument of plaintiff's reasonable reliance). If anything, a person would seek to partner or to transact with Satoshi Nakamoto than with a seemingly ordinary individual who publicly purports to be not Satoshi. Accordingly, it will be hard to establish that the false denial was reliable and caused losses.