My group has a nearly identical problem. We require condolences for cats with the disease we deal with to be sent privately, for a plethora of reasons; yet, for a plethora of other reasons, we do not allow private replies. So we lock topics about cats who have just died, and send a group message asking people to send condolences privately, with the person's email address included in the message.
The problem with the requested feature I see is that it would only work for people trying to reply via email. So I'm not sure it would be effective.
Currently, I just post a message to the group saying we are sorry for the person's loss, and that condolences may be sent to xyz email address.
Messages are the sole opinion of the author, especially the fishy ones.