making an index from custom taxonomies and tags of posts (not hierarchical taxonomy)

I think this is a custom code job.

But here is the query logic I’d use to build this.

1. do a get_terms() for your custom taxonomy.
2. Loop through each term
3. Inside loop do a query_posts() for posts with that taxonomy excluding previous post ID’s and limiting the return to 10.
4. If 10 posts are return then show tax title and then list the ten posts titles using the_title() and the_permalink().
5. Make sure you have a variable that holds post ID of displayed posts so to exclude them on the next tax.

If you need help with code for any of those bits then leave a comment and I’ll be sure to add to my answer.