USB adapters are either using HID-USB interface, or their own communication protocol.
In both cases, it would require protocol reader to be added to the USBloaders, none of the loaders have implemented it.
Only nintendont support HID-USB protocol (in both interface and games).
If USBLoaders ever add support for HID, it will work only inside the loader. No game can benefit from it (I mention it because a lot of users think that working controllers on a loader means working controllers in games)