Not sure if this would quite do the trick or not, but I am using the DG408 MUX to allow multiple inputs on a project of mine, and it is working quite well. The data sheet says that the switching time is TYP 180 ns, which should be well below your requirements. See http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=DG408DJZ-ND for a datasheet. From the datasheet, it appears to allow bi-directional communication: "The analog switches are bilateral, equally matched for AC or
bidirectional signals."
In my experience, the limiting factor has been the ADC time of the Arduino itself, and not the MUX switching time at all.
Hope this helps
Cheers