While I was looking around the HiTechnic website, I noticed a preview link - looks like a solution to the sensors limitation. According to this, the Multiplexor will provide an additional 4 sensors ports and 4 output ports! Click on the link above and then click the 'Development Lab' link.
Also, (great news) legacy sensors and motors (for the RCX) are ALSO supported by the Multiplexor. No price or release date has been set.