It's pretty clear the organizational reasons for doing this, it would be a new gallery paradigm, but very useful.
Like Stephen said, Zenphoto is not currently set up to be able to handle a gallery entirely based on tags. It wouldn't be very efficient or easy, and would require a lot of theme customization and possibly extra programming.
There are people who have succeeded in doing this before, I corresponded with one, Alex Wilson who did heavy hacking on the Zenphoto core back in the 1.0.x days and came up with a great solution: http://alexwilsonphoto.com/gallery/ (Warning: some artistic nudity, NSFW). I can get you in touch with him if you'd like to use his code.
I'd like Zenphoto to at least become more efficient in working with tags in the future, and we've got some changes planned that will help with that, but nothing in the near future. Sorry about that.
For now, we recommend organizing in albums based on your primary interest, whether that be place, date, event name, whatever. Then you can tag and use the current tag features to allow navigation in that way. Good luck!