We've been working on it in recent weeks, and it is already working sufficiently well. There are a few missing bits, not least our release mechanism which has been a bit broken recently. We have a stack of jessie changes to push out too.
We'd like to get a couple more things integrated into the stretch release before announcing it, but if you're feeling brave, you can use the repos at
deb https://repo.bytemark.co.uk/open-source/symbiosis/stretch/latest/ ./
to see how we're getting on.