Testing complex new syntax

[astromog at gmail.com]

I have some significantly extended syntax for Python that I need to
create a reference implementation for. My new syntax includes new
keywords, statements and objects that are sort of like classes but not
really. The implementation is all possible using standard Python, but
the implementation isn't the point of what I'm doing. Speed and having
an extra step to run a program are not issues that I need to be
concerned with.
I'd like to create a preprocessor if possible, because it would
probably be easier than implementing the changes in the interpreter. I
could just drop in standard Python code that provides the functionality
when I encounter a part of my extended syntax. Modifying the
interpreter, on the other hand, sounds like it would be pretty nasty,
even though I have experience in interpreter hacking already.
So my question is: what's the easiest way to implement a preprocessor
system in Python? I understand I could use the tokenize module, but
that would still require a lot of manual parsing of the Python syntax.
Is it possible to use any of the parser module facilities to accomplish
this without them choking on the unknown syntax? Or, alternatively,
would modifying the interpreter ultimately be easier?