I haven't had experience with one-way web service operations and handlers but since http response is send anyway maybe you can use outside filter on all ws calls - then you can measure the operation end time. I've been trying this in my jboss performance tests.
One maybe useful hint - I've encountered many problems during time measurements of jboss webservices (I could not match request and response if there were large number of simultaneous calls - it seemed that some got lost) and finally decided to use client test tool(in my case it was jmeter). This method gave much more objective and complete results.