It looks like some people in the thread below have figured it out — we actually have this. You can start a chat both ways; the system will automatically recognize if this person is on Skype for Business or on Teams.
Thanks and let me know if you have any questions.
This does not work as intended. Even if a user is in Skype for Business only, and I IM them from Teams, it goes nowhere, or they get an email that I'm trying to reach them in Teams.
We really MUST have this in order to start a rollout.
The slash command changes are still in the works. I hope to have more details to share soon.