By saying "current script's absolute physical path", I am guessing OP is referring to the location where the testplan (jmx file) is loaded from. I needed exactly this to generate a CSV file from BeanShell script at the beginning of the run, which is subsequently used in a CSV Data Set Config to read back, so I wanted the script to work just like how the later works when no path is specified. I went through the JMeter source and found this working solution:
import org.apache.jmeter.services.FileServer;
log.info(FileServer.getFileServer().getBaseDir());
I tested this and saw the correct path in the jmeter.log.