I agree that it's good for a DSL to give not just unfiltered but well integrated access to the underlying implementation language, instead of trying to replace it with a dumbed down domain specific language.

It should be possible for implementation language programmers to create new primitives, that visual language programmers can easily use without learning the implementation language.

And that extension interface should be part of the visual programming language from day one, good enough for the visual language to use itself for most of its built-in primitives, not an afterthought nailed onto the side.