That’s a wasted opportunity IMHO, to restrict it so much.
What does the “approved” keyboard look like? It can probably be emulated, unless they have some crypto in there.
In the Avidyne factory-run blog they say all IFD540 owners will receive a free bluetooth keyboard.
I haven’t found any photos, however.
It’s a very positive move.
There is a picture of the likely keyboard (alongside an iPhone for scale) on BeechTalk
And a video is here
That is really nice – many thanks for posting it. I can well see just using the keypad for everything 
The obvious Q… what is the battery life and how does one charge it? Or is there a stow-away holder for it through which it charges all the time?
They don’t actually say this directly but it sounds like the keyboard is integral to the bluetooth functionality approval. IOW, the functionality is not going to be open (well not officially; BT can be reverse engineered unless they put some crypto on it, like Garmin did with the GNS v5+ crossfill protocol, to kill off the Voiceflight voice flight plan entry product).
Avidyne just obtained FAA approval last week (Dec 11) for the release supporting this kb. The restriction to the Avidyne-supplied kb might be related to difficulty getting FAA approval. Here is the Avidynelive thread with the list of other features added with the same software release: IFD540/440 release 10.1.1.0
