> For example, suppose you wanted a sidebar, or an absolute-positioned floating navbar or something.
I suppose that I would have to specify that I wanted to export a ToC with a maximum depth of 1. Currently all my css rules are in a different file (not managed by emacs/org) and I would modify the css in that file to ensure that the ToC appears in a left positioned div, or a floating absolute div, etc.
To be honest, I think that that will only work for a ToC. If you asked "how would you specify a list for the sidebar and another for the top menu navbar" I won't be able to give you an answer[1].
[1] Hey, maybe it can be done, but not as far as I know.