Due to mocl's ahead-of-time compilation focus, function redefinition is prohibited at run-time. mocl does allow user-defined functions to be redefined at compile-time, however, due to technical restrictions, this is not the case for built-in functions.
Based on these technical considerations, closing this as a "won't fix". As an alternative, I will suggest the following:
1) If you have a strong need to redefine standard CL functions, you may wish to obtain the mocl source code and do so there;
2) Failing that, please submit bug reports relating to such functions, and we will try to provide a fix.