So connecting to your zimbra accounts:
/forums/administrators/11171-webdav-briefcase.html#post58137
'anything like it on zimbra':
/blog/archives/2007/08/zimbra_briefcase_1.html
There's also:
Bug 19516 - Ability to segregate Briefcase storage from regular mail storage
Not what your asking but if you haven't seen:
/blog/archives/2007/08/shared_folders.html
Or are you stating that you have an existing windows server share (connecting through samba) that you need to access in the webclient briefcase?
RFE