there is no problem with CIFS over WAN. Copying files from/to a WAN based CIFS share works fine for many years.
But there are lots of problems with the applications. Microsoft Office, especially Word, is not designed to open files on a share with latency +30ms. This will cause a lot of user requests and a lot of broken documents. My proposal: Train your users to copy files locally before opening. Or install a Riverbed WAN accellerator. It will cache all WAN traffic. Or redirect the file access over WebDAV.
I realize now that my question was silly because I thought CIFS worked only inside the same LAN segment. I was asking if there was a way to use CIFS over WAN (without using VLAN's on the comms equipment) but it turns out that CIFS works over WAN by default.