Cat's out of the bag, may as well make public my experiment to enable WebGL on all webviews: https://github.com/rpetrich/WebGLEnabler
That's pretty neat! An unfortunate side effect is that running Mobile Safari unsigned negates the 'dynamic-codesigning' entitlement, which means that Nitro is disabled.
That said, a WebGL-enabled UIWebView running in your own app will also not be Nitro-enabled. I've run the PhiloGL spinning world demo on my device and it's running at 40fps, so simpler demos may be possible without a Nitro to boost things a bit.