Well...you can certainly try to use it, however I don't have a list of window messages that will be useful in any way to you but one - WM_COPYDATA. Shell extension uses this message to send to CH the list of files to perform operation on. However it requires the specifically formatted parameters (I don't currently remember the structure of it - you can check it in the code).
It is being planned to include some kind of real control over the tasks from outside, however I don't know when would I have time to implement it.