How can I allow a masked subsystem to modify the underlying block without breaking its link to a library?

Hello. I have a masked subsystem residing in a library. A callback to one of the parameters of the masked subsystem changes the number of input ports within the subsystem. I have the box 'Allow library block to modify its contents' checked. However, whenever the callback is executed and the number of input ports present inside the subsystem is altered, the block disables the link to the library, and a corresponding warning message is displayed. I am using MATLAB R2011b. How can I allow the block to modify its own contents and still maintain its link to the library? What is the correct approach?

Thanks.