I haven't tested this, so I might be wrong (I don't spend as much time on Mac).
You may be able to route your audio to SoundFlower, then pic this up again in Isadora via the 'Sound Input' actor.
Then try the 'AUDelay' followed by the 'AudioDeviceOutput'.
This should allow you to grab the SoundFlower audio, add a delay and then output to the audio hardware being used.
I can't think of any other way to change the sync of audio/video at the moment.