Yes and no...
With just using the Plus! buildin features, the only thing you could still do is to make a custom personalised status too. And use that personalised status instead of the normal Away status.
The custom status would be like this:
Status Name: <whatever you want>
Name Tag:
Away
Personal message: <none>
Auto-responder: <none>
Show me as:
Busy
In that way your true WLM status is still busy, but to people who have Plus! installed you'll be shown as "Away" in the contact list.
This doesn't exactly provide what you want, but it is the closest you can get if you want custom notifications with sounds, but no sounds when your status is Away.
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If you can script, you have another possebility, and this would be exactly what you want. Only, you need to make your custom notifications within the script and turn of the (online/offline) custom notification in the Plus! preferences.
Such a script can be made fairly complicated (especially if it needs to be user-friendly and take in account Plus! preferences automatically and all), or it can be made pretty simple....
Maybe someone is willing to do it, if you can't...
/ 
