Answering myself for posterity: I forwarded the question to Apple Code Level Support.
Their answer is, in summary: not possible at this time (June '23). The host determines the possible window dimensions, and plugins have no influence over these, nor can they suggest suitable dimensions to the host.
Brambos
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As far as I'm aware AUv3 plugins can not host AUv3 plugins themselves. I have looked into this several times and it has never been possible as far as I know.
Edit: I see you're specifically asking about MacOS. My experience is limited to iOS, where it certainly is not possible.