You won’t find it, for virtually all of Mrs Rhys Davids’ Abhidhamma-related scholarship predates the lapse into aberrant thinking that you describe here.
In 1917 her son was killed fighting in France. The ensuing grief led her to dabble in spiritualism, theosophy and perennialist mysticism, which in turn led to her championing a Vedanta-like interpretation of anattā and positing of a monkish conspiracy to cover up the Buddha’s true message. Her earlier work, however, is not tainted at all by these notions and is about as sound as pioneering scholarship can possibly be.
After I sent my last post I remembered that Mingun Sayadaw also mentions “scholars” (though he doesn’t say whether they be Western or Burmese) who understand Santussita to be female. He himself of course rejects the idea.
From his Great Chronicle of the Buddhas:
Was Royal Mother Māyā reborn as a Male or a Female Celestial Being?
To the question as to whether the royal mother, Māyā, was reborn as a male or a female celestial being in the abode of Tusitā, the answer, no doubt, should be that she was reborn as a male.
In this matter, after superficially studying the Pāli statement mātaraṃ pamukhaṃ katvā some scholars say or write that she was reborn as a female deity; but such reliable works as the Theragāthā Commentary and others hold that “Māyā was only a male deity in Tusitā world of gods.” Concerning Thera Kāḷudāyī’s verses in the Dasaka Nipāta of the Theragāthā Commentary, Vol. II, it is said: dev’ūpapatti pana purisabhāven’eva jātā — “(Māyā’s) rebirth in the abode of gods took place only in the form of a male.”
Also in the section on the Bodhisatta’s auspicious birth, the Jinālaṅkāra Tīkā mentioned:
Yasmā ca Bodhisattena vasitakucchi nāma cetiyagabbhasadisā hoti, na sakkā aññena sattena āvasituṃ vā paribhuñjituṃ vā. Tasmā Bodhisattamātā gabhhavuṭṭhāṇato sattame divase kālaṃ katvā Tusitapure devaputto hutvā nibbatti —
“The womb in which the Bodhisatta had stayed was like the chamber of a cetiya: other beings did not deserve to stay there or to use it. Therefore, seven days after giving birth, the Bodhisatta’s mother died and became ‘son of a god’ in the celestial city of Tusitā.”
Still in the exposition on the Vīsatigathā of the Manidīpa Tīkā, Vol. I, it is asserted:
Sirī Mahāmāyā hi Bodhisattaṃ vijayitvā sattāhamattaṃ ṭhatvā ito cavitvā Tusitabhavane purisabhāven’eva nibbattā, na itthibhāvenā ti —
“Having lived only for seven days after giving birth to the Bodhisatta, Sirī Mahāmāyā passed away from this world and was reborn only as a man (male deity), not as a woman (female deity). It is a regular incident that all the mothers of Bodhisattas should live only seven days after childbirth and that they should all die and reborn in Tusitā Deva abode only as a god and never as a goddess.”
Therefore, the fact that Mahāmāyā was born only as a male deity (deva) in Tusitā should be accepted without doubt.