As you know, Aśvaghoṣa directed some arguments against the Brahmanical understanding of the caste-classes in one of his dramas, the Śāriputraprakaraṇa/Śaradvatīputraprakaraṇa, several fragments of which have been preserved in Central Asian manuscripts and edited by Heinrich Lüders around 1910. It is thus plausible that Aśvaghoṣa dedicated an individual treatise to this topic. The style, the method and the philosophical ressources of the Vajrasūcī, however, are very different from the ones we know from Aśvaghoṣa’s genuine works, and may presuppose Buddhist works such as the Śārdūlakarṇāvadāna if not Kumāralāta’s Kalpanāmaṇḍitikā Dṛṣṭāntapaṅktiḥ. Even if I am not aware of any convincing argument against the attribution of the Vajrasūcī to Aśvaghoṣa, I have always regarded the following statement as anachronistic: dṛśyante ca kvacic chūdrā api vedavyākaraṇamīmāṃsāsāṃkhyavaiśeṣikanagnā*jīvikādisarvaśāstrārthavidaḥ /. “And one observes in some cases that even śūdras know the meaning of all śāstras such as the Veda, Grammar, Mīmāṃsā, Sāṃkhya, Vaiśeṣika as well as [those of] the Jainas and the Ājīvikas.” (*-nagnā- em. : lagnā- Ed.) Although such a list is not per se impossible in Aśvaghoṣa’s time, i.e., although most of its individual elements could have been known around 100 CE, I do not believe that such an enumeration would have been possible, as a doxographic statement, at that time, and even less so under Aśvaghoṣa's "pen." (The absence of the Nyāya from the list is intriguing.)
Another element possibly deserving some consideration is the Sanskrit colophon in which Aśvaghoṣa is characterized as siddhācārya (kṛtir iyaṃ siddhācāryāśvaghoṣapādānām iti), an expression the exact meaning of which remains somewhat unclear to me.
I am looking forward to reading other opinions on this interesting topic.