I've found out that it's very easy to customize UIPageControl page images (I've checked it for iOS7/8):
[self setValue:[UIImage imageNamed:@"my_icon_for_off_state"] forKey:@"_pageImage"];
[self setValue:[UIImage imageNamed:@"my_icon_for_on_state"] forKey:@"_currentPageImage"];
But I wonder can I publish my app with this code, because these variables are declared as private in UIPageControl?
As per Apple, you can't use private API's in your project.
However we can. Don't worry. Just be honest and while submitting the app, inform them that you have used xyz code.
Apple DON'T approve the app where insecure private API's are used.
Many times, I used private API's and Apple approved it.
Honesty is simplicity.