This may be just a fixed expression, but I'm curious what the masculine nominative "einer" represents. Is there another similar expression that uses a mysterious masculine nominative like this?
As Eller commented:
This is not a feminine dative, it is a masculine nominative. Because it is a Indefinitpronomen.
Feminine nominative would be "eine".
In case you are female, you could say "Sieh mal eine an.", it is just very uncommon as it is an idiom known with "einer".
So the "einer" refers to you as the speaker himself, speaking in third person of yourself.
Using this idiom you express some kind of surprise / amazement / astonishment. It could be meant sarcastic, ironic or "real" - and many nuances in between.
So I have to deny an answer to your question about another example of "mysterious feminine", because this is none.
The sentence "Sieh mal einer an." is more like a colloquial expression. The "einer" is not feminine, it's either masculine or even neutral (meaning both men and women).
The full expression means something like "How about that ...". I don't think you can really analyse the meaning of the words separately, because it's an idiomatic phrase and therefore would not work anymore, if you teared it apart.