If you import the “Add a custom layer” sample and the model is unreadable (due to Binarizer not being supported) just right click the modell asset and reimport it. I guess Unity needs one asset import pass to see the OpImport being added by the script, before it can start utilizing it (by which time the model has already failed importing)
Just make sure there aren’t any outstanding compilation errors, I bet it won’t grok the asset import augmentation until after a successful domain reload.