Right, after a lot of getting to grips with the “new world” development and battling some bugs I’ve got a basic version working.
All it offers right now is synchronisation between SmartThings Home Monitor and the Yale Alarm state.
SmartApps can subscribe directly to the Home Monitor state these days, so no workaround required to use a virtual switch to update Yale.
It will require you have somewhere to run the integration now that the Groovy IDE is being decommissioned. It’s currently geared towards using AWS (which is what I use).
This is all the integration does currently - I’ve never needed to display my Yale devices through SmartApps so this isn’t supported right now. Chances are I won’t get time to circle back round to do this, but would welcome any contributions and PRs 
Hope this is useful to people, let me know if you have any questions or feedback!