Thanks Bairen! Will be following the development.
My reading has been: (to save click for casual readers of this thread in the future)
#1 For TF 1.15-2.2 (and 2.3 coming soon), and sometime in the near future, there is no defined & modular APIs to add new device.
#2 It was suggested to directly modify a fork and release one's own TensorFlow package with the device support.
#3 Modular APIs built on MLIR is being actively worked on. At the time of this writing, there is some progress around defining underlying APIs (StreamExcutor, myself not really sure it's relationship with device support though)