well, I found a way to do it - don't know if it is the best way or not. The layer with the cards on it needs to have "Force Own Texture" set to yes. Then you need solid rectangles for the top and bottom areas you want to block out. Then on top of those rectangles you need another rectangle with the effect "Source Out".
see my example: http://www.rieperts.com/games/forum/mask_test.capx
The piggies have Drag and Drop behavior so you can move them around to see how the masking looks...
Note: if your game is spawning new cards on that layer, you will always have to move the rectangles with the source out effect to the top of the layer after creating each card - otherwise the cards will be in front of the mask.