I want to be able to export change requests from the command line.
At least for StarTeam 2009 and earlier, the Command-Line tool (stcmd)
is designed for automating your build process
by allowing you to checkout a set of files in order to build them.
(it also supports checking-in files, but the tool is mostly for checking-out files).
To extract the content of CRs, you can either use a Report (via the UI),
or you can write something in Java that will utilize the StarTeam SDK for that.