I never thought that this would work but it seems to run just fine, putting it here for future reference. This returns candidates numbers that have an application to a requisition that’s posted, used for a T-XML candidate attachment export.

<quer:filtering xmlns:quer="http://www.taleo.com/ws/integration/query">
  <quer:includedIn>
    <quer:field path="Number"/>
    <quer:query projectedClass="Application" alias="sqMotives" preventDuplicates="true" pagingsize="1">
      <quer:subQueries/>
      <quer:projections>
        <quer:projection>
          <quer:field path="Candidate,Number"/>
        </quer:projection>
      </quer:projections>
      <quer:filterings>
        <quer:filtering>
          <quer:includedIn>
            <quer:field path="Applications,Requisition,Number"/>
            <quer:query projectedClass="SourcingRequest" alias="sqMotives" preventDuplicates="true" pagingsize="1">
              <quer:subQueries/>
              <quer:projections>
                <quer:projection>
                  <quer:field path="Requisition,Number"/>
                </quer:projection>
              </quer:projections>
              <quer:filterings>
                <quer:filtering>
                  <quer:equal>
                    <quer:field path="SourcingRequestStatus,Number"/>
                    <quer:string>2</quer:string>
                  </quer:equal>
                </quer:filtering>
              </quer:filterings>
            </quer:query>
          </quer:includedIn>
        </quer:filtering>
      </quer:filterings>
    </quer:query>
  </quer:includedIn>
</quer:filtering>