Handle for ProjectSave

there's no reliable way of doing that. AE doesn't let you know in advance that the project is about to save, and there's no way i know of to deduce that action wtith 100% accuracy. for example, you can check during idle if the project is dirty or clean, and if the status has changed you can tell the project was saved, but you can't predict it beofrehand.

my advice:

use sequence data. each effect instance has an associated sequence data handle that AE stored. AE does so on multiple occasions such as when copy/pasting an instance, however it laso does that when the project is saved. set the PF_OutFlag_SEQUENCE_DATA_NEEDS_FLATTENING flag, and AE will call your effect on these occations with a handle to store data in for saving.