Okay, this is now partly implemented on the git. There's a new file, ssyntax.arc, which contains an 'a-ssyntax and 'a-ssexpand function. Those functions will eventually replace the builtin Anarki 'ssyntax and 'ssexpand functions.
What I intend is that the underlying ac.scm will call the arc-world 'ssyntax and 'ssexpand functions. This means that the user can redefine 'ssyntax and 'ssexpand, and all future symbols,when compiled by ac.scm, will be compiled using that schema.