Me and many others really need this
Luca has been working on general improvements to the cloth sim. Tearing is on the radar, but there are low hanging fruits, relative to that task. Also, Richard Colburn is scheduled to be giving a 30 minute talk about the cloth system on the first day of the Blender Conference. He’s been working independently from Luca, so it’s hard to say what progress he’s made, and how compatible their two approaches would be. However, since Richard work in textiles, it’s likely that he’s not working on tearing specifically.
meanwhile, if you have the money:
it´s a bit of a cheat, of course, but works fairly well.
This would be great for creating some amazing cloth fx work.