Given that there exists no such API, you still could customize the sheet in a non-standard-api-like-way without using private api. The easiest will probably be to observe the subviews of that webview and when one appears (like a popup) check it's class and if it's such a popup, to customize it. Here's how I'd try that.
Still: this is hacky and might easily break in the next update.
Add observation:
[myWebView addObserver:self forKeyPath:@"subviews" options:0 context:@"popup"];
Then observe:
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context
{
if (context == @"popup") {
for (UIView *view in [object subviews]) {
if ([view isKindOfClass: [UIAlertView class]])
[self customizeAlert: (UIAlertView*)view];
}
}
[super observeValueForKeyPath:keyPath
ofObject:object
change:change
context:context];
}
Then do your customization in such a method:
- (void)customizeAlert:(UIAlertView*)alert { ... }