If we want to support DI, rather than just a "class" it should probably be an ObjectFactory spec.
Topic on Talk:Parsoid/Extension API
Noted. I'll update this detail in the second pass. We might have to fix our code first and / or wait till we start moving Parsoid extension implementations out of Parsoid codebase and into the extension's repos.
Scott's latest patch (in review and soon to be merged) supports this spec.