I have proper instancing now working. The user-facing API is decent, and the drawcalls should go in as is too.
The custom vertex attrib part is a bit hacky, needs discussion.
As usual with SVN being a PITA to work with client-side, and me posting a huge blob benefitting nobody, patches will be posted in pieces once the earlier ones go in. The full set can be viewed at the instancing branch at github.com/clbr/seirr.
More on forum.
Against 4527.
Less hacky VAs by using the enum from the FVF branch.
GL part.
Will be available in shader-pipeline.