I wrote an optimizer rewriting the AST to implement various optimizatoions:
https://bitbucket.org/haypo/astoptimizer

To be able to use it, or use any other kind of AST transformation, I propose to add a new sys.setasthook() function to set a custom AST hook.

Attached patch is a proof-of-concept. The patch should be improved: document the function and add write unit tests.

Prototype of the hook:

def asthook(ast, filename):
   # ...
   return new_ast

I don't know if we need to add a compiler option (like _ast.PyCF_ONLY_AST) to skip the hook.