I think it might be useful to provide a mechanism for users to set build properties (properties that are usually set in the build.[username].properties file) for a configured SDK. The properties would then be passed when ant targets are executed.
These properties could be added and edited in the same screen that's used to configure the Plugins SDK. They could be available just in Liferay IDE to prevent any conflicts or unintended consequences or users could have the option of having the properties written to the build.[username].properties file on the file system.