It would be useful to be able to get an AudioProcessorARAExtension out of an AudioProcessor without requiring a dynamic_cast by adding a virtual getARAClientExtensions function, similar to how AudioProcessor already has getAAXClientExtensions, getVST2ClientExtensions and getVST3ClientExtensions. For one thing, it would allow cleanly implementing AudioProcessorARAExtension in a subobject of one’s audio processor in addition to the (currently only) option of making the audio processor a subclass of AudioProcessorARAExtension. (my personal motivation is actually just avoiding the dynamic_cast, but the above motivation is an even better one 
).
To achieve both motivations, this can be implemented similar to getVST2ClientExtensions and getVST3ClientExtensions, doing a dynamic_cast and triggering an assert by default.