Errai 4 templating and data-binding work with any native JsType that wraps a DOM element. You could start using Elemental 2 wrappers with Errai UI or Data Binding right now if you wanted. This design was intentional so that you can use any DOM wrapper library (or even custom DOM wrappers) so there is no incompatibility.
What you can't do currently is inject an Elemental 2 wrapper without writing your own producer. We will look into this once the new version becomes more stable.
Good to know this is on your radar, and yes, the Google guys are not as fast moving as the awesome Errai team :-)
My concern was mostly for Errai classes that inherit from your hand-written JsInterop DOM wrappers. If you decide to switch over
to Elemental 2 DOM wrapper for base classes, this would be a breaking change...