I am using the same method as you for script library. I used the method described by Kerry Doan at http://www.doan.me/script-library-in-soapui-free.aspx
When i tried to access this script library from project load script i did not have access to the testRunner so i created the testRunner object and as i did not have access to the context object i had to create that too.
Check out the code below.
import com.eviware.soapui.impl.wsdl.testcase.WsdlTestCaseRunner
import com.eviware.soapui.support.types.StringToObjectMap
import com.eviware.soapui.impl.wsdl.testcase.WsdlTestRunContext
//to create the testRunner object I need a testCase object and a new StringToObjectMap
//I initially used the test case in the script library but as i needed the
//testRunner object to the test case being executed so i created the object to
//the test case being executed.
//I did not want to worry about test suite or test case names so i used their index
//assuming that there will be at least one test suite and at least one test case when
//this script is run
testCase = project.getTestSuiteAt(0).getTestCaseAt(0)
tcRunner = new WsdlTestCaseRunner( testCase, new StringToObjectMap() );
//A context is essentially a WsdlTestRunContext object and as you can see below all i
//have done to create that is pass it a test step object which was obtained by using index
//rather than name.
tStep = testCase.getTestStepAt(0)
tcContext = new WsdlTestRunContext(tStep)
//my script library is in a seperate project called `Script Library` and all the
//groovy scripts are in a test suite called `Script Library`
scripts = project.workspace.projects["Script Library"].testSuites["Script Library"];
scripts.testCases["Scripts"].testSteps["runTest"].run(tcRunner, tcContext);