You don't have to intrude new syntax to create an embedded DSL, that's the whole point of an embedded DSL, it uses the languages existing syntax. Smalltalk and Lisp are full of DSL's, as is Ruby, of the three only Lisp has the ability for syntactic abstraction, every Smalltalk DSL uses native syntax. See Seaside's DSL for html generation or Glorp's for database mappings.
I don't think you can introduce new syntax in Python and have it run as part of the language, so magic methods, decorators and metaclasses are as good as it gets. You'd have to write a parser to handle new syntax, and that makes it external, right?