Ori, I’m replying here with an alternative, even though you may be happy with the icon-setting solution on tiddler save. Depending on use-purposes, you – or others – might prefer a css-only solution, as described below:
A stylesheet can be set to display a visual icon (or virtually anything else you can imagine) before tiddler links (which affects title display in story river if you use setting to display tiddler titles as links). You can use filters (including tag-based filters) to choose which tiddlers get styled in which ways…
The basic stylesheet and explanation is here: http://linkstyle.tiddlyspot.com/ (Note that one can specify not only what’s before the link, but also the link styling itself, and also what might come after the link.)
To see wikis where tag-based icons are shown in this way (without using the icon field), check these out:
The first of these is rather old, and the refnotes plugin has been much developed since then, but it was my first attempt to implement a tag-based linkstyle solution, and includes a bit of an explanation of how it works.
Cheers!
-Springer