Zotero's reports are very useful, but they are unfortunately a bit inflexible. So, I hacked together a script to give me a little more control. In addition to letting you modify the data included, it also inserts all the styling as inline CSS, so you can send a colleague your pretty report without having to include seperate stylesheet files.
If you have any problems or suggestions, feel free to let me know; my contact information's on my blog. View the source code here.
Thanks to Peter Baumgartner, there's now modified version of this tool for the German-language localization of Zotero: Zotero Bericht Anpassung.