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seems to me you're going to have to go pretty deep into the bluetooth protocol.. your computer will say it's a computer when it connects on bluetooth - not a keyboard. You'll have to make it pretend it's a keyboard, I have no idea how you'd do that: and I'd imagine it's a bit outside the realm of Max/MSP - although nothing seems to be out of the realm of Max MSP really, at least not on this forum.
I do know that bluetooth devices communicate a list of 'services' they offer: you may be able to find a way of having your computer add the keyboard 'service' so your Ipad allows it to connect as a keyboard.
I imagine that those people that actually develop bluetooth keyboards need to do that to test their stuff: you may be able to find something if you look at bluetooth developer forums.
That's just me thinking along, I actually have no experience in this area...