I've been struggling with extending or overriding a Sapphire class, FileIFrameField. After a few hours of research, I got on IRC and confirmed with 'simon_w' that this isn't possible.
However, it occurred to me today, would it be possible to put the entire FileIFrameField.php in my 'mysite' code directory, nested the same as Sapphire and override it. Similar to Zend.
I'll give it a shot, but wanted to post in case anyone had any further thoughts/advice.