If I understood correctly, the pointer is just a field in a common struct (common in event-handling code). As he mentions, it's usually used to point to extra data that you allocated separately in case you need it for that specific type of event, but what if you're having trouble with allocation and your data conveniently fits in the pointer field itself? So he just cast his controller id into the unused pointer parameter. He's not mangling any allocator data.
Which, if you're not going to use that pointer parameter otherwise, I think is perfectly sensible (and as long as you don't have code along the line of if (ptr != NULL) free(ptr);). I do it all the time... cough. For example, when creating a thread, you can pass it a void* to some data for it. I just cast a threadID integer into there. Hell, you'll see just that in the first example on a google search for "pthread example"!