I think you're absolutely right that you would only have to worry about the hub shifter, because you could just use a Shimano derailleur with the STI shifter to handle the cassette (because SRAM's cassettes are spaced to the Shimano standard). I have no idea if the shift ratio is the same for a front STI and a DualDrive hub though. It would be really interesting if it turned out to be. Good luck!
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In memory of Jim Price (aka. sydney) ...