In regards to by XDSb portion of my question, I found this video from Mark Bolinsky that states that in order to compress existing streams, you just write a routine that reads then saves each existing stream.

https://www.youtube.com/watch?v=ZiGrlslPpQU&t=380s
 

https://docs.oasis-open.org/imi/identity/v1.0/os/identity-1.0-spec-os.ht...