The problem with that is that it's nothing like CPDLC in the real world and having to get your clearance from the PMs loses a lot of your immersion in the sim. Personally I just think that it's tacky, annoying, and I cringe hard whenever I get a PM with one of those, usually departing OMDB. It's also a bit nonsensical because it assumes that all aircraft are datalink-capable - when I fly IFR in my C206, it most certainly is not... but that doesn't stop a controller from hitting the button and sending me a "datalink" clearance. Hoppie's system (www.hoppie.nl) is the best thing for datalink on VATSIM, but unfortunately it is used by so few controllers and even fewer pilots. Moreover, datalink clearances are primarily meant to reduce frequency congestion; the controller should pay just as much attention to aircraft regardless of whether the clearance is being passed by voice or datalink. Nonetheless, I'm really excited to be able to use CPDLC properly with POSCON.
That being said, shortened readbacks are a bad idea and you honestly won't find such a procedure being commonplace in many places around the world. As a controller, one has to ensure that the clearance is received correctly and the only surefire way to tell that, short of waiting and seeing that the pilot is flying the wrong heading/SID, expecting the wrong runway, climbing through his cleared level, etc; is through receiving a readback of the clearance in full. That is, a readback that includes every bit of information (not necessarily every word, lol) that was included in the clearance. Otherwise, all I'll be doing is asking the pilot to "confirm X, Y and Z" until they do. /my2cents