I found the solution, following up on Sally’s initial suggestion there is a nice filter to check for email validity, wpcf7_is_email.
A custom version of that lets me say that the special email tags are valid.
Thanks again, Sally!
A quick report — it seems to work, but contactform7 complains loudly and displays error indicators in a few places because the email address specified as a tag [whatever] is reported as a malformed email address.
That sounds like a great solution Sally, I will try it!
Thank you.
