It is possible to glue ceramic or stone directly onto the Flexpren waterproofing? 

Yes! We did the pull-out tests taking into consideration the NBR 13755 standard for facades (CERAMIC FACADE AND EXTERNAL WALL FINISHING WITH THE USE OF COLLECTING MORTAR). This standard requests a load of 0.5 MPa in a sample with a diameter of 50mm, and in these tests the results were excellent.