c=all is only available to admin, unfortunately.
But you could make a plugin with this functionality. News does this however already, so you just tie in list.tpl to news and you're done.
But this is meant to be copy&pasted to your programmer.
edit: yes, it's possible. I for one have a search engine done that does only tags (before the tags were made in cotonti, it was running on seditio) so it should be easy. The same needed request is here, programmer.
You could learn programming by yourself (cheaper and takes more time) or hire a programmer (expensive, but faster).
If you need I can recommend you to some.
<a href="http://www.domenlo.com">Surreal Art</a>