here’s a long shot: is there any way to allow an AWS account to create an API Gateway custom domain using another AWS account’s hosted zone?

checkout AWS RAM, you can share critical resources across orgs and accounts using this.

huh.. it doesn’t work.

Cannot create ResourceShare: UnknownResourceException: Organization o-xxxxxx could not be found.

You can do it manually like this https://serverless-stack.com/chapters/share-route-53-domains-across-aws-accounts.html