You can use a simple rule triggered by Thing status changes and update an Item, right? That’s probably going to be way easier than mucking with the REST API form a rule.
You could also use executeCommandLine to interact with the karaf console.
If you have a lot of Things to map to Items, you can use Thing Status Reporting [4.0.0.0;4.9.9.9] to make handling that a little easier.
Finally, in JS Scripting and jRuby (perhaps others) you have access to the ThingRegistry. If you have the ThingUID (which you need to use the REST API too) you can get at the status directly.
if( things.getThing("shelly:my:thing").getStatus().toString() != "ONLINE" )