For a mirrorless system with APS-H sensor not to use EF lens would be a total folly on Canon's behalf. With the EF-S mount there are only a few good lens (3 or 4?) whereas to have to redesign all the good EF lens for a new mount does not make commercial sense
Using EF-S lenses would surely mean that Canon would be unable to reduce the flange distance and thus unable to save much space on a mirrorless body as there rivals have.
Seems most likely to me that they'd create a small new system of general purpose mirrorless lenses along with an EF adaptor to allow for more speicalist lenses to be used with AF intact. Not being able to use EF-S lenses doesnt seem like much of a problem to me since there arent really any speicalist lenses in the lineup besides arguebley the 60mm macro which has many EF alternatives anyway.