Putting a toc in the sidebar used to work, but was broken by commit 9652cdfe2eb16150518e34af33c8858118fe0a09, which, in turn fixed a bug with the toc not appearing during page preview.
So, if toc is a sanitize hook, it can't be used in the sidebar, because the sidebar is only added to the page later. If the toc is a format hook, it shows up in the sidebar, but not at page preview time (because format hooks are not called during preview). Also, calling the toc as a format hook makes any headers that are hardcoded into the page template show up in the toc, which is rarely desirable.
I can't think of a way between these that works in all cases. Maybe call the format hooks when generating a page preview? Maybe add an option to toc to make it embeddable in the sidebar?
Hmm, I think I need to call format during preview. Another case is that inline uses a format hook to insert the inlined content..