I'm mentioning it here as I posted this as a bug fix / feature request on GitHub (https://github.com/playframework/playframework/issues/4924#event-370978560) and had it summarily dismissed with the comment that it needs to be "vetted" as actually needed by the group here.
The work-aournds (kludges) I have to date a complete crap (mutable code being a no no and all...) -- I merely wanted to get it functional so i could work on the balance of the API for a project while I figure out the better (best) way to handle this situation.
Looks like I'll have to write the layer myself for something that perhaps could have been included in the initial OAuth class design as a default parameter. (With some "connection" between the signing of the retrieveRequestToken and retrieveAccessToken methods versus the OAuthCalculator used after the initial handshakes are done).
I have no idea how to get things done within the community but I do believe the OAuth class would serve better if augmented to include these signing methods:
https://github.com/mojodna/oauth-signpost/tree/master/src/main/java/oauth/signpost/signature
Much appreciation to the people that provided the twitter example that I started working from: https://www.playframework.com/documentation/2.4.x/JavaOAuth
(Sorry I have no way of knowing which repository is actually "official" as there are so many of the forking things...)