I'm a little confused by your statement "would like to use the scheduled callback queue assignment to route scheduled callbacks to a different queue.". This isn't how Dialer is ordinarily set up, so I assume you are configuring the callback as a standard (inbound) callback for the secondary queue? This would explain why Dialer still tries to call the number, since it is not aware of the callback.
I'm not sure why marking the number / contact as uncallable in Dialer is affecting the (essentially unrelated) callback in the second queue. I think we would need to see more configuration details.
I like Robert's idea, although I'm not sure if he is proposing a second campaign, with it's own contact list and the secondary queue, or inserting an inbound callback. One of the problems with the latter is, if the call is still not answered, the system will not automatically retry. It also doesn't use any of the Dialer dialing modes, it essentially becomes a Preview call.
HTH
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Paul Simpson
Views expressed are my own and do not necessarily reflect those of my employer.
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