It would be nice to think that there were some Mahayanists camping out at the Mahavihara, but it's unlikely.
The historical development of Buddhist ideas is quite dynamic, much moreso than is often commonly acknowledged.
When what is written in the Patisambhidamagga is taken in _context_ of all the works it is attached to in the Tipitika and it's Commentaries, especially the Commentaries on the books of the Abhidhamma, it is pretty clear that it is meant to mean empty of own being.
This is a Mādhyamaka interpretation.
The Mahavihara Buddhists were very hardcore. And they believed without a doubt, that ultimate dharmas really do arise.
The Paṭisambhidāmagga wasn't composed at the Mahāvihāra. Moreover, even during the later period of Indian Buddhism (after the Mahāyāna was established in the sense that we know it today), there were many fully ordained Theravāda monastics who accepted the Pāli Tipiṭaka and who also accepted the Mahāyāna teachings. The Chinese monk Xuanzang (7th century CE) met Mahāyāna Sthaviras (Pāli: Theras) at Bodhgayā (1000 monks in one monastery), at Kaliṅa (500 monks in 10 monasteries), at Bhārukaccha (300 monks in 10 monasteries), and at Surāṣtra (about 3000 monks in 50 monasteries). Those at Bodhgayā were living in a monastery built by an early king of Sri Lanka. He also described the Abhayagirivihāra of Sri Lanka as being a Mahāyāna Sthavira monastery.
In fact, it's likely that all Hinayana schools did, because realizing the emptiness of persons is predicated on accepting both ultimates that arise and illusions or concepts based on those things which do not arise.
No it isn't. The emptiness of persons is not predicated on any such thing. Otherwise no Mādhyamika or Yogācāra would be able to realize the emptiness of persons.
The emptiness of all phenomena, well that's totally different. Trust me... Hinayanists were not trying to realize the emptiness of all phenomena.
This is inaccurate. There are Hīnayāna texts which teach the emptiness of all phenomena.