I've learned [here][1] that there are two ways to modify an `\item`:

 - `- [@123]` will set the counter of enumerations (or "ordered lists") to 123. 
 - `- [$Rightarrow$] :: bla` will generate a "description".

Both do not really yield the flexibility that I was hoping for in my question, but it seems that this is as far as one can get. 

  [1]: https://www.gnu.org/software/emacs/manual/html_mono/org.html#Plain-lists