redefine extern(C) function in a shared library

[timotheecour]

Is there a way to redefine a extern(C) function in a dynamic load 
library? (platform is OSX if that matters).

In a normal D program, I can usually redefine a dynamic load 
library as follows: eg, for _d_assertm:
extern(C) void _d_assertm(ModuleInfo* m, uint line){...}
which allows to do stuff before exiting; there are many other use 
cases.

In a dynamic load library, redefining this (or other extern(C) 
functions) doesn't seem to work: the new definition is ignored.

This probably explains the problem I had here:
http://forum.dlang.org/thread/tddzekqtgkqhwjiaknte@forum.dlang.org#post-tddzekqtgkqhwjiaknte:40forum.dlang.org