At that point, wouldn't the approach be to defend from the client side? Namely, we can instruct the client to not trust any content sign by such-and-such keys. This can be done by pushing out a certificate revocation, etc.
This would be pretty cool (remotely revoking signed exchanges), however it's not part of Google's proposal - Unless every previous security consideration about caches is accounted for in SX's, it's probably not safe to start faking the URL bar.
Certificate revocations do apply to signed exchanges.