Unless I'm mistaken, there is no way to show 23.976 fps in the time display. There is no preset and if I enter a decimal point in the custom time display, it will round to 23.
Please tell me what you would recommend for coordinating this frame rate in an audition session. If it's not possible, I recommend adding this time display to your release.
Unfortunately this isn't possible in the Mac Public Beta. I'll add your notes to the feature request. Previous versions of Audition didn't support this frame rate (it should've IMO). Adobe Soundbooth, does currently support this framerate, however.