This is, for the present, a single patch against the Subversion version control system. This patch allows you to use the svn:keywords property to define custom keywords. For example, if you set the svn:keywords property to Ringlet=Id, Subversion will expand the $Ringlet$ keyword just as it would normally expand $Id$.
This is by no means a complete or recommended change; there are ongoing discussions about better ways to handle custom keywords. Still, for the present, it does the trick for me.
Comments: Peter Pentchev <roam@ringlet.net>