So it turns out using Bluetooth keyboard+mouse is possible with MorphOS on the Mac mini, a thread at Amiga News.it has more details. There is also a video on YouTube showing a Bluetooth mouse being used with MorphOS.

I'm typing this post on my mini with my Apple Bluetooth keyboard, there is an Apple HID device listed in Poseidon, it has two interfaces for keyboard and mouse, so in practise it works like a standard USB device.

Thanks to Capitanvideo for discovering this.