Nicely done!
How hard would be to add a GPS based beacon?
Keeping track of exactly where my dog is within 150ft radius, not so much. The "leash" analogy is also entirely misplaced. "Leash" implies having control over your dog, so it can't get away. With playtag it seems, there's not really any element of "control" at all, and if my dog gets more than 150 feet away it's basically useless.