There is currently no easy way in the API to access to the "sonar.projectBaseDir" property value from a sensor. The FileSystem::baseDir() method returns the baseDir of the current module. Whereas when you're in a multi-modules project, it should be possible at any point of time to retrieve a file/report based on this project root directory.
Workaround: inject ProjectReactor in your Sensor constructor. Then use ProjectReactor::getRoot()::getBaseDir()