I went ahead and created a Zulip chatroom for people to ask asynchronous questions on workshop presentations (I intend to use it for my own ML workshop presentations, but if someone from another workshop has a use for it, please feel free).
https://icfp2020workshops-unofficial.zulipchat.com/
Registration should be open to anyone. If you are an author of a talk and would like to receive asynchronous questions there (in addition to, not replacement of, ICFP’s Q&A mechanism), please just create a topic for your talk in the appropriate stream. (To create a topic just start writing a new message and edit the topic name.) Let me know if something doesn’t work!