I don't think there is anything built in to SVN that will do this for you. This isn't a common use case because concurrent editing is handled gracefully by SVN, and therefore isn't considered to be a problem worth notifying other developers about.
If you want to be notified when a developer starts editing a SVN-tracked file, you'll probably need to develop a mechanism yourself. You could have a script running on each developer's machine that every 5 minutes does a "svn status | grep ^M" (or whatever the Windows equivalent of that is) and then sends the resulting text output to a (custom) server. The server could then compare the text received from each developer machine, and if it differs from the text received previously from that machine, it could email the other developers to notify them about the change.
That seems like a lot of work for not-so-much benefit, though. I also wonder if the developers might find that too intrusive.