The only other thing you need to do is enable the "custom" notification itself (which is not enabled by default).
You can see the "#custom" commented out in /usr/share/byobu/status/status.
You'll need to enable that in your ~/.byobu/status file by copying (and editing) the notifications you want from /usr/share/byobu/status/status in ~/.byobu/status. For your custom notifications, just remove the "#" from "#custom".
Everything you did here is correct.
The only other thing you need to do is enable the "custom" notification itself (which is not enabled by default).
You can see the "#custom" commented out in /usr/share/ byobu/status/ status.
You'll need to enable that in your ~/.byobu/status file by copying (and editing) the notifications you want from /usr/share/ byobu/status/ status in ~/.byobu/status. For your custom notifications, just remove the "#" from "#custom".