> You just need a custom :sitemap-function.
> The default `org-publish-sitemap-default' flattens the hierarchy.
> 
>  From the manual:
> 
> ‘:sitemap-function’
>       Plug-in function to use for generation of the sitemap.  It is
>       called with two arguments: the title of the site-map and a
>       representation of the files and directories involved in the project
>       as a nested list, which can further be transformed using
>       ‘org-list-to-generic’, ‘org-list-to-subtree’ and alike.  Default
>       value generates a plain list of links to all files in the project.
> 
> You may create a function similar to `org-publish-sitemap-default' but

> using `org-list-to-subtree' instead of `org-list-to-org'

Thanks for the pointers, but I can't figure out how to get from there to 
a toc? As I understand, `list' is a list of files from my project and 
`org-list-to-subtree' does add stars to this list. But how do I get all 
sections and subsections from my file to appear here?