The restriction on executable pages is enforced by the kernel at the process level. This particular mechanism won't allow for enforcing diffeerent policiees for UIWebView code running in the same process as app code. If UIWebView could run JavaScript code in a separate process like Chrome then it might be possible.
Doesn't the UIWebView do that for you? And isn't it written by Apple? Why couldn't Apple make it use Nitro? Not sure where the security risk would be.