I am exploring the feasibility of building a simple application that
can run on a PDA/phone and emulate a HID (Human Interface Device) over
its bluetooth interface (or, USB).  In this way, a nearby PC could
recognize it as a keyboard or mouse instantly without the need for new
drivers or for building a client/server pair. The purpose would not be
malicious, but rather to aid in accessibility with special keyboard
layouts and mouse movers.

Some phones seem to support this natively, but I need a solution that
could run on any device, such as an iPaq running Windows Mobile, or
even another desktop PC with full Windows. Is this limited by the
bluetooth hardware, or is there a way to programmatically expose the
HID profile/service to other devices in range?

There is a bluetooth service that performs old-fashioned Serial over
bluetooth, but on the receiving PC end, it is not translated into
keystrokes or mouse movements without a 3rd party program such as
AACKeys.

The web seems dead silent on this topic, so I was wondering if anyone
had any ideas.