Well, one thing that we've found is that the Laird unit ships from the factory with the persistent storage keys already set to allow for easy setting the HID mode. The generic ones that I've bought in Japan (close to a dozen at this point, from different manufacturers) have also had the same keys, but the generic ones from Europe don't seem to (just a sample from 2 different users on the TREG slack).
Theoretically we should just be able to re-write the correct persistent storage keys onto the units that are missing them, but that research is still ongoing. As of now, we've only been able to do this with the Laird device or with the ones from Japan that ship with the same keys.
The BT900 seems like a module for hardware developers to include in their projects. It doesn't seem like a standalone dongle. Maybe I found the wrong thing; can you send a link?
As for what the dongle will work with, it should work with anything that would accept a USB keyboard, as that's what it looks like on the host side of the connector.
I'll let @wmertens share the link to the project, as it's his project. I was just the one who figured out how to get it working in the first place.