I don't know if it's an option for you, Leonard, but in this situation and with these requirements I'd personally be tempted to modify the ChucK source to support something like a --capture command line option which records all output and then auto-closes the audio output file when all shreds have finished.

The problem here is that the duration has to be known beforehand. The batch conversion can't know how long these scripts are, and the scripts themselves shouldn't be rigged to put that information somewhere else. I'm amazed how difficult that appears to realize. I went through the docs and I can find nothing of use.