Storing AEGP data in a comp or project?

well... there's no facility designed for that, but you can make use of layer and comp comments which are i've NEVER seen a user make use of, and are very rarely even displayed in the UI...

i've used that trick on numerous occasions where i needed to stash some layer/comp related stuff. you can just hope the that user doesn't use any other plug-in doing the same stuff.

alternatively, you could add a null layer to the comp, name it "my stuff, do not delete" and lock it (just make sure the user understands what it is). then use that layer's comments field.

another solution would be to create a folder in the ae project (name is something clear) and put folders inside it who's names contain comp id data to tell what comp they refer to, and the data itself. that will create minimal clutter in the project. (just one visible folder for the entire project)