Axel Davy continues to be the developer leading the Mesa and Wayland work on supporting PRIME for GPU offloading in an effective manner. Experimental support came last August and the work continues to be revised with the previous round of "request for comments" being in early January.
With this Wayland GPU offloading support the specifying of a secondary GPU for rendering is still done via the DRI_PRIME environment variable. Support is being worked on for GPU offloading in two ways: rendering to a linear buffer shared with the compositor or rendering to a tiled buffer and then copying the contents to a linear buffer shared with the compositor. This latter approach to GPU offloading will yield better performance for gaming.
For many more details on this Wayland PRIME work, see Axel's new mailing list thread.