I was having the exact same issue with my mock REST request / response in Soap UI 5
The other answer is correct in theory, creating a response in a groovy script does in fact work. However, there was some issue with the Groovy OnRequest script. Here's the code segment I used to create a valid XML response (courtesy of DivxYoda from this question HERE):
com.eviware.soapui.impl.wsdl.mock.WsdlMockResult mockResult = new com.eviware.soapui.impl.wsdl.mock.WsdlMockResult(mockRequest)
def httpResponse = mockRequest.httpResponse
httpResponse.setContentType("application/xml;charset=utf-8")
httpResponse.writer << "<root><user>ABC</user></root>"
httpResponse.status = 200
return mockResult
Tested, and works great for me!