I have a production server still running zarafa, with the original passwd plugin and that still works.
I installed the plugin by dducret, because that is a fork of that original passwd plugin, which I assume has the changes you have made. The problem is not so much that it does not work, but that it does not even seem to get registered.
I do not see it in the list of plugins. I also have not found any way to debug the webapp to the extent of registering and loading plugins.