When "use_locks" is on the tool should still check the "Allowed" header or the "supportedlocks" property in order to find out whether locking is supported by the server or not.
sorry for this late answer. But I did not know there is this forum on our sf.net page. Seems you are the first one to discover and use it.
Meanwhile I added an OPTIONS-request and check for the DAV compliance class. Only when this is 2 locks will be used. I think this should do the task, as class 2 servers must support locks, allthough there might be class 1 servers that support locks too, but this seems to be very unlikely.
I know of one server (box.net) that claims to be class 2 and does not support locks. But this server is not able to respond to a HEADER-request correctly, so I doubt it will know about Allowed and supportedlocks.