As an musician I often exchange created files with my friends and because of the slow asynchronious connection transferring needs some time. But when I want to listen into the file before it is already complete downloaded Foobar won't play it?
The handle is not locked, other players like Winamp oder WMP can play the song, if I copy the incomplete file and try playing this part, it works.
Is that a problem of the foo_mp3 plugin or can programming a little plugin solve this problem? It's really annoying.

It's a design decision which is not likely to change. If you want to listen to an incomplete download, just copy (Ctrl-Drag) it to get a new copy to listen to.