I had the good fortune to meet Wolfram Schneider at ECDL 2006.
He has done some great work with Z39.50 searching and also library mashups.
He built his app quite a while ago - probably it was one of the first advanced Z39.50 apps on the web, I don't know.
His web app is called ZACK Gateway, it does a Z39.50 federated search (what librarians, always needing to be different, call "broadcast search") across a wide number of libraries.
ZACK Gateway (in German, for Germany)
It is particularly specialized for locating books in Germany, but it will also check a small number of libraries worldwide (he has better data for Germany).
Here's a map for The World is Flat (it will take a moment to load).
He has also done a mashup that displays the distribution of MARC and other formats (Z39.50 target types), by catalogue type, worldwide.
ZACK Z39.50 target maps (select a target type to get a map)
I wonder if maybe he could work with LibraryThing - I'm going to suggest this.