https://webaudio.github.io/web-audio-api/#AudioWorkletProcessor-methods says:

may have a valid static property named parameterDescriptors which is an iterable of AudioParamDescriptor ...

but https://webaudio.github.io/web-audio-api/#dom-audioworkletglobalscope-registerprocessor step 8 has:

If descriptors is neither an array nor undefined, throw a TypeError and abort these steps.

Though it does not define what "an array" means; is it in the sense of https://tc39.es/ecma262/#sec-isarray or something else? That would allow Proxy to Array; at least Gecko's impletation at the moment does not, as far as I can tell.