Can a predicate adjective be in a prepositional phrase?

No, a predicate adjective can't be in the form of a prepositional phrase because an adjective can't be the object of a preposition. An adjective can be part of a prepositional phrase when it is describing the noun object of the preposition.

Example: Mary is in a big hurry. (the object of the preposition is the noun 'hurry', the predicate nominative; the adjective 'big' describes the noun 'hurry')