The browser engine implementing the client side of this is open source. Even the attester could be open source (because good crypto doesn't need obscurity). Also the server side can be open source. It will still be a DRM and you still can't do much about it, because you can't force server/service operators to use the opensource software in a way you want. So if they want to enable WEI, they can do it. No matter if it's proprietary or open source. Actually it's even easier when it's opensource.