I've heard back from the development team that this feature will not be implemented at this time. The current functionality will allow you to accept emails from a user alias on a domain alias. For example, if an email alias is created on the primary domain, emails can be successfully sent to UserAlias@DomainAlias.com as well. However, at this time, it's not possible to send from a user alias on a domain alias.
At the moment, the Send From dropdown contains the primary account and any domain aliases, SMTP accounts, and user aliases that have Send From functionality enabled. If there were multiple entries listing each user alias at the different domain aliases, a user's Send From dropdown could grow quite large - even when just a few user and domain aliases have been configured on the domain.
With this functionality not implemented, I would encourage you to continue utilizing your workaround from version 15.x.