I think the problem is related to how the CanvasStroke works. Included in the stroke info is the state of the modifier keys and this seems to override the IKeyPress. So, if the stroke you are using already has Alt+Ctrl+Shift held then all will be well. However, you still seem to need the IKeyPress.
A solution that works rather better is to use CanvasClick to specify the beginning and end of the stroke. You do need to know exactly where the model is for this, of course. This shows the rough idea:
[IKeyPress,SHIFT+CTRL,[IClick,Brush:ClipCircle]]
[IKeyPress,ALT+SHIFT+CTRL,[CanvasClick,[IGet,Document:Width]/2,[IGet,Document:Height]/2,[IGet,Document:Width]/2+200,[IGet,Document:Height]/2+200]]