Did you find a solution here?
I posted a new post yesterday quoting yours - it’s an old one so I didn’t think it appropriate to continue your thread but here’s my post http://forum.mikrotik.com/t/bridge-filtering-client-to-client-traffic/171162/1
We can do L2 NAT so I think it might be possible to pull some kind of trick. It’s important because IOT devices often use unsecure UPnP discovery methods and we need to be able to satisfy their functional needs whilst preventing them from doing anything else.