Official PlasticSCM Support
In this thread I mention a PlasticSCM plugin that was written in 2013, and that may have worked for quite some time. Evaluating the code for it's adapter and model, it doesn't appear to be too dissimilar from SVN and Git's SCM.
From what I can see, this plugin seems to be very close to working, and could even be made official with little effort.
I am willing to sponsor the work to get this plugin working again, or integrate it officially so that we can properly use PlasticSCM within Redmine.