Given the examples
- He went back to Santa Monica, which was his hometown.
(The comma is necessary for a non-restrictive relative clause)
- *He went back to Santa Monica, where was his hometown.
(ungrammatical because of where)
- He went back to Santa Monica, where he was born.
(grammatical despite where),
your theory is partly correct.
Yes, the locative relative pronoun where can't be the subject of a relative clause, unless there's also a locative predicate in the relative clause; a location can be the subject of a locative predicate:
- Santa Monica, where stands the famous Bridge Over Troubled Waters
But not normally.
However, in (3), where is not the object of be born, which is intransitive and can't have an object.
Even if the clause is transitive, its object is unlikely to be a locative:
- Santa Monica, where I spent Hanukkah
Where, when, why, and how are adjectival or adverbial in nature, and don't normally function as noun phrases, which is the minimum necessary for a subject, direct object, or indirect object. The relative clauses these wh-words introduce tend to modify words that refer to the same kind of adverbial:
- the reason why
S ~ the time when
S ~ the place where
S (but not *the way how
-- how can only be used without antecedent or wh-word ~ how to do it or the way she did it)