Thanks for posting. This sounds like a good idea, my only concern being that in a crowded plex, too many icons might cause confusion as to which thought they belong, particularly if tag icons were the same size as thought icons.
The trouble with tag names is that they easily become too long for a string of them to display conveniently in the plex, which is why I turn tag display off in my Brains. However, if it were possible to display only a tiny icon or symbol without a name, this might be more appealing.
I seem to remember that someone in the past tried differently colored wingdings as names of tags to add a bit of variety, but soon ran into problems because, unlike thoughts, you can't have two tags with the same name (pity). This idea still works as long as each tag has a different wingding for its name - can be any color you like.
If you use icons that much, why not use an icon editor to create a system of icons with unique designs and colors to suit your system? Then a single icon might be able to contain elements of two or more icons, enough information to satisfy your need.
Another solution might be to use PB's inbuilt Capture Thought Icon feature to take a screenshot of a number of icons placed in a group, and use this screenshot as your main icon for that thought. You can copy and paste this image anywhere, just like any other icon.