In order to add preprocessor capabilities you could use PPJASS.
It's an existing wrapper over PJASS and has the following keywords:
#define <defname> <value>
It's also called via command line and works like the C preprocessor. It runs the preprocessor results through PJASS. The error lines returned by PJASS are translated back to the original line numbers (before includes got resolved) in the file.
I'm sure you can find the thread where it was posted with the search function.