I'll pass on some things I discovered, in case anyone has this question or anyone can add to this.
I succeeded to some degree, by doing this:
1. Installed Soundflower; this creates virtual sound inputs and outputs, so sound can be passed between applications.
2. In GarageBand, I created a real instrument.
3. Set input to Line-in
4. Added the Voxengo Delay AU (
Precise audio delaying plugin (AU, VST) - Voxengo Sound Delay - Voxengo) which is free and simple. The Apple AUDelay repeats the sound as well, rather than just delaying it (unless I missed something).
5. Set GarageBand's sound output to Sountflower, and turned monitoring on.
6. In QT player, new movie recording, with audio input set to "Soundflower".
The good news is that I can save the GarageBand document, which can be opened and everything is immediately running. But, it would be nice if there was a way to add an effect to any input directly so that I only have to use one application.
Any suggestions to streamline this? It is for an installation which will be run by students, so the fewer steps needed to set this up the better.