Hey CodeG.
There is no way to store JSON in the Dgraph. What you can do is create on your application a parse to that part of you doc to String format and then save to an Edge. Then when needed return this Edge to JSON in your application level.
I did not understand that part. Are you creating unique predicates for each device?
Also you could use with that parsed doc @Facets Get started with Dgraph
Cheers.