So, I ended up using these in a 4chan quest. The Metabolic Extension Controller is used by the Omega in order to bypass Divine punishment, and keep high-value soldiers functional past the point of death.

MEC troopers are converted into "heavy MECs" (see thread's first picture) when tissue repairs become unviable.

http://suptg.thisisnotatrueending.com/qstarchive.html?tags=left%20beyond Find the quest log here.