Apple responded to my feedback via Feedback Assistant essentially saying they are not going to do anything with my feedback requesting 3rd party app access to Siri Voices. It appears that this response may be double-speak.

I received a message from a developer that the latest betas of iOS 16 and macOS Ventura, Siri voices appear to be unlocked for all apps. The developer also said Apple may lock them again before the final release, so let's not count our chickens before they hatch. The developer and I think some hope is warranted though.