The 6704 is not supported under DAQmxBase. You can submit a product suggestion but I highly doubt that it will ever be supported. It is currently only supported under Traditional DAQ. Which means future DAQmx / DAQmxBase support in Linux is unlikely.
The 6704 is supported in the DAQmx API in NI-DAQ 7.4 which means you will be able to use these boards under Linux once DAQmx is supported under Linux. Howerver, as of today there is no Linux solution for that particular board other than to write your own RLP driver.