Custom content on all author pages?

What’s the best way to insert custom content into all author profiles (themename/author.php), regardless of which theme is in use?

Specifically, I’d like to display a link to another system on our intranet, but only on author pages. I didn’t see any likely hooks for doing this.

Barring that, is there a way to perform a custom action only on author.php pages?

In the worst case, I can install JavaScript that detects “…/author/username” in the URL and springs into action, but that’s a bit of a hack.

Thanks!