The phrase already has the opposite meaning. It's used primarily in political contexts, where someone admits to an unsavory position for which there has been a less unpleasant alternative reason. For example, citing a racist reason for a policy, when the tacit agreement among proponents was to not say that "out loud".

It is good policy to be explicit when you should be explicit. But as a political strategy there is a reason not to say the quiet part out loud, at least not yet.