1. I think you don't need a 3rd party tool. Go in unity conncetion Admninistration>>User, Select a User, go to Edit >> Message Action, select "Accept and relay the Message" for the message type you need.

I set for me the relay and accept options and set as notification devices mobilphone and SMTP.

If I got an voicemail the unity call my mobile phone, set th MWI to ON and send an email. It's to much I know, but it's only for test purrpose.

2. I don't know if it possible that the use turn relaying on and off. But test the personal transfer rules for user. You must enable it for the user at Unity Connection Administration >> Class of Service, Select the Class of Service which the users are members, Check the box "Features >> Allow Users to Use Personal Call Transfer Rules". At this time the user can login to personal transfer rules at the unity connection user page entry.URL is: http:///ciscopca

Maybe at this way is a possibilty to turning relay on or off.

Sure, you could rig some sort of notification with notification device techniques but I read the question as more of asking what is the inherent capability associated with using the relay parameters.  The best way I can sum this up is to refer you to this blog that my colleage, Bill, wrote:

http://www.netcraftsmen.net/resources/blogs/voicemail-only-voicemail-relay-and-integrated-messaging-options-with-unity-connection.html?blogger=William+Bell

It gives you the in's and out's of the Relay Only vs. Store and Forward options of CUC including how MWI behaves.

Hailey