Any of the other component classes. The simpler ones are maybe PersonalTools or Logo. In principle you just need to return the HTML code of your component. The question is, where to get the HTML for the TOC from. For that you will need to search the MW code base, i.e. find where/how the TOC is built and where (if anywhere) it is stored. If it is stored somewhere, great, just grab it. If not, call the method, that builds it. You will probably also need to suppress integration of the TOC into the page text.
In your component you can get the Skin object by calling $this->getSkin(). From there you can find your way to a number of other relevant object, e.g. for the OutputPage use $skin->getOutput(). Just saw, that OutputPage has the methods enableTOC() and isTOCEnabled(). Maybe search from where these are called.