bbPress.org

I found at last what I was looking for:
https://github.com/turtlepod/gt-bbpress-author
here is a “GT | bbPress Author Drop Down” that works like a charm!
It works for me in BuddyPress, with bbpress forums, WP 3.5.1

I could not find the plugin in the repository, just in github.

For non-specialists:
You have to copy the file content (that you see on the github page) in a new file, save it as a php, put it in a zip file on your desktop, and then install the plugin by using the “uload a zip” feature in your wordpress installation.