Yup. So for Requests at least, the fix is easy: because OpenSSL kindly just quietly ignores cipher suites it doesn't know about we can unconditionally add it to the requests/urllib3 cipher string. In the first instance we'll just do it statically, and then we can consider down the road whether Python/cryptography could give us a way to ask whether we should prefer ChaCha20 over AES-GCM.
In the short term, my expectation is that we'd still want to prioritise AES-GCM over ChaCha20 in Requests: is there any reason to think that I'm wrong there?