At the park is where she grabbed my attention.
I don't see a problem with the PP "at the park" being the subject here. A similar attested example is Under the mat is the place where we used to leave the keys, where the PP "under the mat" is subject.
The expression "where she grabbed my attention" is a noun phrase (not a clause) in a 'fused' relative construction in which "where" is interpreted as "the place where", i.e. the place where she grabbed my attention.
The noun phrase is functioning as subjective predicative complement of "be"
Incidentally, the above analysis shows that there is no inversion involved.
Note that we could have an interrogative tag, "isn't it?", where "it" is anaphoric to "at the park", showing that the latter is subject.