Hi,
I think the best way to do this is to create a new extension, that defines its own input type, which also adds on to the $sfgJSValidationCalls global variable. See the Semantic Google Maps code for how an extension can define a new SF input type. Instead of calling validate_field_type(), though, the new validation call should call some custom Javascript function that you create - you should add it to the $javascript_text returned by the input type. That way, you shouldn't even need to modify Semantic Forms at all. I hope that all made sense...
-Yaron