Hi Guy, I am planning on making a "Tapered extrude" type mechanism here pretty soon, but for the moment the method that Steve describes is how you can do it.

See here for some examples:
http://moi3d.com/forum/index.php?webtag=MOI&msg=941.5
http://moi3d.com/forum/index.php?webtag=MOI&msg=941.6

The video that Steve is mentioning is here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2662.6

- Michael