From what I saw, this is not possible at the moment. Correct me if I'm wrong there or if there is already an issue tracking this.

Lets say we have a my_image.png that is processed with an CompressedImageSaver to a basis universal texture file. At the moment, the saved file inside the imported_assets directory sill has the png file extension although the file is not an valid png file anymore (because its now a basis universal texture). Of course this makes loading the files simple, since the asset_server.load call can just take the source path and lookup the same file inside the imported_assets folder. If we want to change the path of the processed asset, we need some kind of lookup table where we have an unprocessed path -> processed path mapping.

One potential way of doing this would be to store the processed file path inside the AssetMeta file. Lets say we have our assets/my_image.png and we process it with the CompressedImageSaver to imported_assets/my_image.basisu (I don't know what the proper file extension for basis universal is, lets pretend its right), we could also save the imported_assets/my_image.png.meta file. Every time we call asset_server.load("my_image.png"), we could now look up the path .meta file, and we could add a processed_path field there that points to the processed path ( my_image.basisu in this case).

I'm happy to discuss other possibilities there, this was just my initial thought how to do this. I would love to write an implementation for it, but I thought it was important to first discuss roughly what it should look like.