Store a tree structure per user OR use taxonomy privately?

Hi,

I am trying to make a multi-user site where most nodes stay private to the author. I want each user to be able to organize their nodes in an outline/hierarchical form, privately, but I can't figure out a good solution for this. Here are some things I have thought of:

1. Use the private taxonomy module. This makes terms private to the user. The downside is that you choose a vocabulary to be private, so all users' terms are stored in one vocabulary. Thus, functions like taxonomy_get_tree() return an enormous data structure, the whole vocab, that includes everyone's terms. Maybe I could add a function that duplicates taxonomy_get_tree() but only for the users' terms?
2. Give each user a unique vocabulary. At user creation, a new vocab would be made for them, and I could alter autocompletes and such to only grab terms for their vocab. Then taxonomy_get_tree would be specific to the user. The downside here is that my permissions administration page would soon grow to enormous lengths because each vocab is listed on it.
3. Use the book module? I do not know much about this.
4. Store a tree structure in the user object. I don't want to write my own tree traversal/editing/etc code, though having the structure stored in the user would be nice.

Any help would be appreciated, I am stuck!

Thanks,
Jim