New nest device type with external temp sensor (st814)

I’ve been trying to use the keep me cozy II and it really is just lacking in the interface department.

There isn’t a clear way to set the target temp that is straightforward to find for the casual user. It doesn’t support sensing presence, doesn’t show up in smarttiles and a few other things.

I was thinking of taking the nest device type code and extending it to reference a st814 temp sensor as well. Replace the current temperature set points and store it in the st814 while making the nest’s set points indirectly controlled by the st814 as the keep me cozy II smartapps does.

A couple of questions before I start on this:

  1. has anyone done this and I’ve just missed seeing it?
  2. is it even legal/advised/OK to have a device type referencing another device?
  3. any other thoughts, suggestions, concerns?

I’ve also been thinking of writing a new device type for the st814 that has the thermostat capability, too - for a different place - but I haven’t gotten very far thinking about that yet.