This was a problem that was fixed in Mendix 7.7
The release notes of Mendix 7.7 say: "We fixed the result of an import action where the root of the import mapping is an (inheritance)
element. This no longer gives an empty result."
I tried out your example in Mendix 7.7, and it works.
Do you tested the webservice if the answer of the webservice call is correct?