Sorry, I push the wrong button when I reply

I'll try your configuration, but
- where are files stored? (I dont't see server B in your post)
    

Read http://www.infrae.com/products/tramline
There is description where files are stored

  

- it seems I'll lose Zope User Restrictions
    

Possibly not. This is application server that decides if user
is allowed to upload/download file (by setting proper headers).