Actually, it would be quite easy to implement, but that’s probably a different feature request.
Basically, your client could keep track of who you have invited to chat and who you share a group with and it could automatically throw away any and all emails that arrive coming from outside this flow (i.e., such as via a channel). A huge drawback would be that normal people couldn’t contact you via a vanilla pgp vcard either, so it’s not acceptable.
A different workaround would basically boil down what we have recommended multiple times here an in the other topic: creating a new key pair for each group and channel (either completely independent ones to maintain anonymity or subkeys to prove that you are the same person). We might reuse the same single chatmail account address for multiple keys in such cases when it becomes a scarce resource later, but that’s a technical detail. Also, the user interface could completely hide such (sub)keys and just use it for sending, reception and ignoring replies under the hood.
As a completely different alternative, perhaps we could introduce subaddressing to chatmail accounts and you could vary the nonce suffix of your localpart between each group, each channel and each direct message so that nobody could contact you without your consent even if they know your single key pair. You would probably still have a main suffix that you distributed over vcard files and QR codes privately for compatibility with vanilla encrypted email that you could rotate if you ever get spam.