As I understand it that's not possible at present I'm afraid, though I've passed on the request for you as it sounds like a useful addition. Not sure if redirecting the diff tool will work, the only workaround I can think of offhand would be to delete the contents of the file you want to overwrite (or delete it and leave an empty placeholder file in its place) before merging.
I'll update you when we hear back regarding the query, though it may be a while I'm afraid.
I believe we're waiting on an update to Subversion before this can be implemented as it's currently not possible in the JavaHL back end. The features needed in order for this to work have been requested though.