Instead of having only a subset of the valid entities, why not supporting the whole specification?
It seems to be available as a json file, which is easy to reuse: http://www.w3.org/TR/html5/entities.json

It is true, though, that this would make the compiler much larger, which is not good when using it client side...
But for client-side usage, perhaps we could use the browser, which is supposed to know all entities (creating a cache of entities progressively, when required, and asking the browser when the entity is not in the cache).

But perhaps we don't need that for now...