Is this subject complement a predicate nominative or predicate adjective?
At the park is where she grabbed my attention.
Note: At the park is a prepositional phrase that functions as a subject.
"Where she grabbed my attention" is a nominal clause. It would therefore function as a predicate nominative and not as a predicate adjective.
Note that this interpretation requires the prepositional phrase "at the park" to function as the sentence's subject, as you recognized. However, prepositional phrases don't usually function as subjects. Therefore, many people will prefer to construe "where she grabbed my attention" as the subject and "at the park" as a predicate adjective. This word order (predicate adjective--simple predicate--subject) can be found in many other contexts, e.g.: "Blessed are the meek."
At the park is where she grabbed my attention.
That sentence contains a be verb inversion. The normal order is: Subject Verb Predicate.
The normal order would be:
Where she grabbed my attention was at the park.
The subject is the entire noun clause: Where she grabbed my attention.
at the park is part of the predicate.
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.