There is a so-called Lufthansa "jumpseat pass", which costs somewhere around $30, but I've never been able to find a gate agent who knew what it was, although a buddy of mine at Comair swears by them.
All I know is thaty he went through the Delta International gate agents to purchase it. Was a hassle, but he did it multiple times.
SOME european airlines have jumpseat agreements- I could have jumped on LUXair if I had stayed with my previous carrier, but not with my current cargo carrier.
Not everyone has access to ZED rates, either. While at DCI, fer instance, I couldn't ZED myself to Belize (before they started flying there) on American, but had I worked for Continental, I could have.
Another option would be to get to Frankfurt or somewhere close, then rail it to Croatia. I rode eastern Europe's rails back in summer 2000, and it was great- cheap, quick and efficient.
Also, for instance, I do remember that as a DCI pilot, we were allowed to fly in back of South African and Air Morrocco (or whatever)- I'd check with your company jumpseat rep and see what kind of options you have. They do exist.
Have fun in Croatia!