I have made a new version of the OCIO config which uses the native colour space of each DRT as the shaper. So, for example, the FilmLight T-CAM (Rec.709) display transform converts first to FilmLight E-Gamut / T-Log, and then applies the DRT as a 3D LUT to that. This means that it handles values which are outside AP1, but inside E-Gamut, and therefore works on imagery with or without the gamut mapper applied.
It is currently only in my fork, but I will submit a PR later, so it can be properly added the main OCIO configs repo and the super-repository.
I’m not sure if is mentioned elsewhere, but the additional display transforms show up when using the version of the ACES 1.2 config in the feature/aces_vwg_gm branch of the repo.