For a current project, I was thinking of implementing WebDAV to present a virtual file store that clients can access. I have only done Google research so far but it looks like I
Bit late to the party, but I've implemented most of the webdav protocol and I can tell with confidence you'll need to implement most of the protocol.
For OS/X you'll need class-2 WebDAV support, which includes LOCK and UNLOCK (I found it particularly difficult to fully implement the http If: header, but for Finder you'll only need a bit of that.)
These are some of my personal findings:
http://sabre.io/dav/clients/windows/ http://sabre.io/dav/clients/finder/
Hope this helps