It seems that many of my assumptions about a Bluetooth driver for the Newton were correct and it is indeed possible to implement such a thing. The basic idea is to implement a new comm tool based on the TAsyncSerTool which would provide the RFCOMM layer. This is very similar to the work I've done on Nitro, it might even be a bit cleaner ... and run on on older MessagePads as well.
One common low-level interface to Bluetooth cards is a UART. This means that we can simply talk to the card via a serial channel. There are different kinds of UARTs - if a card would use a UART the built-in TSerialChip driver won't recognize, we would have to implement a new TSerialChip driver. But that is fortunately not required, the Newton seems to recognize most UARTs just fine.
The next layer is the Bluetooth HCI layer. It is a set of commands and events encapsulated in data packets. It provides functions to establish connections, do the key and PIN code negotiations, transmit data and so forth.
On top of the HCI layer goes the L2CAP layer. It provides logical connections over a physcal HCI connection. This will be the basis for the RFCOMM layer.