Hello Richard,
I assume you want to modify properties of a block on 'initialization of simulation'.
You have callbacks on various blocks which can be accessed by right-click "Block properties" and under callback functions.
Adding respective block property set/get parameters will enable you to control pre-simulation/post and during simulation actions.
For example the following shows how to add a callback for MaskInit http://www.mathworks.com/help/xpc/driver/create-initialization-callback.html
