How do I programmatically update the Post-Build Command Tab of the Build Definition

I'm not sure whether what you are looking for is officially considered public API, but refer to this blog post for a technique to find the API on your own: http://ryehle.wordpress.com/2011/11/21/finding-the-right-service-api-for-your-process-extension/

The Plug-In Spy points me to HighPerformanceAgentPostCommandLineConfigurationEditor, and in that class I see BuildAgentConfigurationElement.POST_BUILD_PROPERTY_WORKING_DIR, which has a value of com.ibm.rational.buildforge.buildagent.postcmdline.workingDir.

I could be wrong but I don't think that is exposed to the SCM system. I wanted to update the duild def for something as well and gave up.