How to use AssetDatabaseExperimental.SetImporterOverride automatically before import?

I'm trying to replace and extend the PSDImporter with my custom version by following the documentation. However, the example only shows how I can use

AssetDatabaseExperimental.SetImporterOverride<PSDImporterOverride>(path)

to set the importer manually via a menu item. Instead, I would like to apply this importer to all PSD/PSB files in the project, so I thought I could simply call the same thing in OnPreprocessAsset, but this doesn't work.

Is this a bug or how would I go about this?

 

I still have the same question, even though the API is out of experimental, if anyone knows. ^^