Whilst I wouldn't rule anything out (there's some very clever bods with n900's in their hands), I'd be very pessimistic about expecting wine to run anything well on any ARM based mobile right now, but especially an audio sequencer that depends on sound running flawlessly.

This should be possible but isn't guaranteed for any particular device to work. Read some of the HostMode threads here on TMO to get a feeling for what is involved and pros/cons of plugging a USB device into your phone.

You know you could 'split' the headphone out to give you two connections to the same audio output - I know it's not the same as being able to monitor and output separately but it depends on exactly what your use case is to tell how important that is. Don't know if you want to monitor and play separate audio (e.g. preview something on headphones whilst playing something else out) or simply be able to hear what you are playing (same audio on both).