Speaking as a compiler writer, I would say the implementation
issues are minor, compared to everything else we already support.
The key issue in my view is the Ada design philosophy. Extensible syntax
is inconsistent with the principle that Ada programs are intended
to be readily shared, read, understood, and enhanced by other
programmers. Extensible syntax is great for a "personal" programming
language, but not great for a "group" or "team" or "enterprise"
or "international" programming language.
For what that's worth...