OTOH, Valve typically has been very amenable to fan material before. If Valve was approached as a production partner, with limited oversight over production, one might even be able to encourage them to chip in as a publicity event. Keep the same director, writing and special effects designer as the production leads, let some valve employees chip in additional writing and or settings/special effects.
the bigger question in my mind is "Is there enough plot to Portal to tell a full length movie?" for all that the game was amazing, the plot basically is, wake up, get tested, break free, kill GLaDOS, escape. Not a lot of room there for dialogue, plot twists, or character development. This clip worked on the basis of capturing the ambiance of the portal universe, the dystopia and the feel of relentless monotony. not sure that would work for a feature length film.