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:
\hardware\energia\msp430\variants\[existing-model]\
\hardware\tools\DSLite\common\targetdb\devices\[existing-model].xml
\hardware\tools\DSLite\common\targetdb\options\[existing-model]_GNU.xml
\hardware\tools\DSLite\common\targetdb\options\[existing-mode