The PluginManagerDecorator is supposed to be used for extending the PluginManger.
This is made fairly difficult by breaking API compatibility between the two interfaces.
PluginManager accepts a plugin locator for init, Decprators do not.
PluginManager has different defaults for init than the Decorators.
PluginManager returns a list of plugins from loadPlugins, Decorators do not if they override the function.
I have only taken a short look at yapsy, so I cannot say that I have looked very much further after finding these, but I must say that from an OOP standpoint, this is very strange software design (that would partly not even be possible in strict languages for good reason).
I hope there will be an effort to keep an clean and consistent API in the future because the project looks quite useful to me.