As a form of documenting... I just added another new MCU to Energia and -in short- had to do the following to get it working:
In Energia 1.8.7E21:
Copy from a close family member the following files and replace the model name with the new model name in the directory-/filename and file content:
Add the new model to the following files:
Of course, check for differences between the two family members (e.g. missing or added peripherals, memory sizes) and edit the copied files accordingly.
After the above changes it might help to remove the temporary build tree. These are located in ~/.energia15 for Linux and %userprofile%\appdata\local\Energia15 for Windows.