And if Mallory captures their computing devices, you don't want him to be able to forge messages using their private keys that make it look like they said something they didn't.
Then I guess you don't want to be using OTR then, because this is one of its features.
From the OTR site:
The messages you send do not have digital signatures that are checkable by a third party. Anyone can forge messages after a conversation to make them look like they came from you.
This is actually what gives you deniability. After the chat is completed, anyone can forge additional content. This means that no particular piece of content can be proven to be a part of the original conversation.