Francesca Zambello famously stated that operas should be cut to last an hour. Schlingenseif, if given the chance, would probably do the same (considering his "dramatic vision", he might as well). There is a reason why the composer sends a finished manuscript to the printers, namely, because the manuscript is finished. While the composer himself can go back and tinker with it if he wants (Wagner and Bruckner both did), the idea of having someone not connected with the composer "finish" or "add" something to the work is nonsense. There are notable examples of this (Mozart's Requiem, Mahler's 10th, and now Holst), that succeed (or fail) to varying degrees. But can't we agree that the composer is king as far as his own work goes? Anything else is a farce, or worse, a debasement.
Terry, of the Contrapuntal, thunders forth this pronouncement. For the record, Frau Mahler approved of Deryck Cooke's performing version, though he never called it a completion, only an orchestration of Mahler's finished four-stave sketch. Frau Mozart asked our buddy Franz to finish K.V. 626. There is a history of completions or performing versions, otherwise approved by those who knew the composer best. Or at least went to bed with him.
Also, Herr Schlingensief would never cut an opera to an hour. To do so would deprive the audience of his "brilliance." If anything, he'd strap the audience in the stalls and run through the performance again, shouting after every dramatic turn, as a perverse apres coup: "Did you get it? Huh? Huh? Did'ya?"