I can define the property sonar.groovy.codenarc.reportPath property in my pom.xml to point to an existing CodeNarc XML results file. But the code to read the XML results file is not correct. The value of this property is used to get the project property.
In the file GroovySensor.java at line 110:
File codeNarcReport = getReport(project, codeNarcReportPath;
File codeNarcReport = getReport(project, GroovyPlugin.CODENARC_REPORT_PATH);