As noone has responded to my problem placing a default value in case of a non-existent tag, I found my own solution for it:
I created my own tld and corresponding class, adding parameters like baseKey and defaultValue. It was pretty easy: Just copy and past the existing struts implementation and add the parameters to the tld and their handling in the corresponding class. I've spared you the sources here, but I might give them to you if you ask by sending a mail to me (not the mailing list). Lukas