Comment by mst

I regularly add Symbol based features to JS libraries I'm using (named methods are riskier, of course)

    import { SomeStreamClass as SomeStreamClass_ } from "some/library"
    export class SomeStreamClass extends SomeStreamClass_ {
      [someSymbol] (...) { ... }
      ...
    }

I have not blown my foot off yet with this approach but, uh, no warranty, express or implied.

It's been working excellently for me so far though.