We should probably have a hook for this case, too. BEL doesn’t come with any data but we could pass the tab title to the hook context.
I found it particularly useful to be able to identify the tab (mailbox) where new mail arrives by implementing this feature. If that feature is implemented, it would fulfill my above desire. It seems concise and clean. When new mail arrives in aerc, the tmux window displays a bell notification. Not being able to identify it in a subsequently activated aerc tab would be particularly inconvenient if the user is managing a large number of mailboxes.