You are talking about the .NCX file, I guess.
I don't think you can do that. I found that ereaders are rather strict on the NCX and OPF file.
You could experiment a bit with the navLabel entry, fi. a blank <text></text> part, or maybe even with the <content... part.
But I don't know for sure.
You know about nesting the navPoints ? You can make a two or three level TOC that way, having each next level "explode" on a click.
Of course, you could make your own toc.html, apart from the NCX. Then you are free (within XHTML/CSS) to build whatever you want...