I'm wondering if it's possible to assign tags to images,etc that are hidden so they don't display in the tag cloud & image/album pages etc. Anyone have an idea or general direction to look for something like this? I was thinking of the custom_data plugin, and then not calling that on the theme page itself, but haven't checked to see if custom_data is searchable or not...

I don't think this is possible. The easiest would be to take the functions envolved with tags and make custom functions from them that exclude tags from display. Either as custom theme functions or as a custom plugin.

Hmmmm.... A project down the road then.