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.

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.