I see you say here “The RRT will be made fully invertible, and that the various sweeteners will be moved to a default LMT.”
When the RRT is made invertible would that mean that one could then use that as an input transform without getting illegal values over 1 as in now the case?
In this particular case here (with the Digital Emily textures), where I am reading in an albedo texture map made in sRGB primaries ljnear space into ACES, in addition to not wanting shifts in colors, I would also like to be able to read it in without having the RRT tone curve crush the blacks. This seems to be an issue when reading color textures made outside of ACES that the RRT filmic tone map changes them in undesired ways. A filmic tone curve is very desirable when viewing a render, but it is definitely not desirable to get crushed blacks on an albedo texture. We might say
“Friends don’t let friends view scene-linear without an S-shaped viewing transform, and friends don’t let fiends crush the blacks on their textures.” 
For now the best workaround I have for that is to preprocess the image through an inverse of the display with an added matrix to keep the resulting pixels below 1, and then read that into Utility-sRGB-texture space for texture painting and rendering. That works pretty well. But the possibility of a fully invertible RRT might open up better options.