Hello ted. I think it's acceptable to ask to see additional photos but in the age of paid social media, it's probably the best etiquette to ask WHERE you can see additional photos. If she can't direct to to a source (clips, OF, LF, etc), if she is interested in such a session maybe she will take the initiative to send you some.

My best guess if a Dommes isn't showing her feet, she either isn't aware of the value of her feet or she thinks they are not marketable. Of course, it's possible she doesn't like having her feet worshipped but that's rare.