<p>My company has a subsidiary with a slow Internet connection. Our developers there suffer to interact with our central <a href="http://en.wikipedia.org/wiki/Subversion_%28software%29" rel="noreferrer">Subversion</a> server. Is it possible to configure a slave/mirror  for them? They would interact locally with the server and all the commits would be automatically synchronized to the master server. </p>

<p>This should work as transparently as possible for the developers. Usability is a must.</p>

<p>Please, no suggestions to change our version control system.</p>

You should try
The SVK version control system
SVK
is a decentralized version control system built with the robust Subversion filesystem. It supports repository mirroring, disconnected operation, history-sensitive merging, and integrates with other version control systems, as well as popular visual merge tools.
On this link there is text about
Using SVK to Synchronize SVN Repositories