Following this guide and substituting
AMD, I've been able to get my mid-2012 MacBook Pro Retina functional again, albeit without the advantages of the discrete GPU (background: original GPU failed sometime between 2013 and Oct 2017 but machine continued to function and I had no reason to check for the GPU until Oct of 2017, Apple replaced logic board Jan 23 2018, second GPU failed most likely on last Friday evening and machine refused to boot at all).
The only problem, or at least the only one that I'm going to worry about, is that things are running a bit hot. Is there any way through software that I can either power down or throttle the GPU? At least I assume from the information in that guide that it is the GPU causing extra heat due to there not being any drivers loaded to control it.