Turns out that mail to example.com is not automatically sent to the bit bucket, although that would presumably be up to the sending system. My test message from Gmail end up being processed normally. The domain doesn’t receive anything, so Gmail told me it’ll keep trying to deliver it for awhile.
This shouldn’t really be a problem, though, because the hashed “addresses” are already blacklisted, so no mail is being sent to them anyway.
I wish I had the time and existing knowledge of phpList plugins, this seems like a good plugin (“Hash Blacklisted Addresses” …?). That way those who don’t want the functionality wouldn’t be troubled with it, but those who want (or feel they need) it could optionally install the plugin. I could figure it out but by the time I got it ready the GDPR would already be in effect.