We are delivering a very basic TinyMCE skin with Grappelli which fits the needs of most use cases. While there are requests to add certain features but we will stick to the basic version for reasons of maintainability.
You can add custom TinyMCE-JS by using a Media inner class on your ModelAdmin (see https://docs.djangoproject.com/en/dev/ref/contrib/admin/#modeladmin-media-definitions).