No they can't: part of a process cannot create executable memory while another part cannot. They'd have to move WebKit completely out of process: a huge task as it already does most text drawing.
An interesting alternative would be to enhance the current 'Save to Home Page' feature beyond specifying a manifest of resources and a home page icon. If this was fattened up to allow developers to make something that looked even more like an app but was running Safari (not a UIWebkitView inside a native app), you might have something interesting.
