well, there are streams you can add to a layer, but these are some very specific streams.
this group of streams are referred to as dynamic streams.
strokes from the paint tool and text animators are such streams.
i think you can make them hidden, but i doubt any of these will solve your problem of associating data to a layer.
arbitrary data can be added to a layer.
you can put an effect on the layer and keep your data there, or write your data as text to the layer comments, or... write you data into a layer marker comments... or... i have no more ideas as how to keep the data with the layer.
alternatively, you can keep the data for all layers at some other place, such as with the AEGP itself.
in general, there are two ways of looking at this problem.
1. the data needs to keep across sessions.
2. the data is temporary and can be regenerated if needed.
case 1 is problematic.
look at this thread:
http://forums.adobe.com/message/2837630#2837630
it talks about how to save non-layer effect data with the project.
for case 2, i'd go with keeping the data with the AEGP.
just keep each layer's id, and it's comp's project item id, so you could identify each layer's saved data.
