Resolution:
Subversion (aka SVN) was not supported by TIBCO Designer version 5.5 or lower, as a Revision Control System (aka RCS).

Starting from TIBCO Designer 5.6.0, TIBCO introduced a new feature called the scriptable RCS Adapter. This adapter allows you to integrate TIBCO Designer with your preferred RCS, that is not offered “out of the box” by TIBCO.

The scriptable RCS adapter forwards TIBCO Designer RCS operation requests to a user implemented script that must conform to TIBCO Designer scripting protocol. Please refer to "Tibco Designer User Guide -> Chapter 3: Working with a Revision Control System -> Scriptable RCS Adapter" for details.  

In general it is the user's responsibility to implement this script. In the particular case of Subversion on Unix however, the documentation does provide a sample script for SVN's integration with Designer.