How to install a taxonomy system that represents records in another high-volume, non-content-node table?
You don't specify Drupal 6 or 7. If you can, use 7. Then you can do the following


Have your content (gossip posts), as
nodes.
Put your celebrity names as taxonomy terms
Apply whatever fields you like directly to the taxonomy term. No need for attached nodes or custom content.

There are several auto tagging modules out there, Open Calais is free and would suit your needs. You could even just use the 'Person' results from it to only tag celebrities. 

Drupal 6, did have some issues with very large taxonomies (over 100,000 terms and 2,000,000 individual tags). D7 has sorted most of these, so it is unlikely that you will have to go down the custom code route.

If you do you will not only need to sidestep the node system, but also the taxonomy system and mysql, so it is not as easy as creating a custom table.