Disclaimer: Bluetooth MIDI is not officially supported, but can be utilized by connecting via third party apps. The steps below will show you how to connect Bluetooth MIDI through Apple's Garage Band software. A wired MIDI connection is still strongly recommended for the best experience.
In order to get the iPad app to detect Bluetooth MIDI, please follow these steps.
1. Open Apple's Garage Band on your iPad.
2. Tap "Keyboard".
3. Go to Settings in the upper right corner (gear icon). Then tap Advanced.
4. Tap Bluetooth MIDI Devices and follow the steps to connect.
Once the keyboard is connected through Garage Band, you will then be able to select the keyboard as an option in the Playground Sessions app.