In any case it’s unclear to me if the webservice ever provides this information, or if it is just included in the obspy.client interface for future use or manually adding.
if you have a look at what IRIS FDSNWS serves
(http://service.iris.edu/fdsnws/event/1/query?eventid=4768345&includeallorigins=true)
you'll see that the GCMT solution is not included in the list of origins
for this event (and I think the GCMT catalog is the only one that
includes focal mechanisms / moment tensors). So this is not a problem of
the obspy client.
I'd recommend asking at webservices@iris.washington.edu.
The IRIS DMC's FDSN event service will not return any focal mechanisms, either for the GCMT catalog or otherwise. The GCMT focal mechanisms are available, among other locations, the GCMT Project website (http://www.globalcmt.org/) and also the DMC's data product archive (http://www.iris.edu/spud/momenttensor).
Managing parametric event information is not a primary function of the IRIS DMC, so do not count on ever being able to access a rich set of event information from the DMC's event web service. Instead we suggest that you go upstream to the catalog creators such as ISC, NEIC, EMSC, GCMT Project, etc.
in case it still matters, the ObsPy dev version can read the NDK files from the GCMT project which includes moment tensors. They can then also be saved as QuakeML files.