hey, i was just noticing that this system was updated 4 days after the last post in the thread. thought i'd mention it.
actually i found another way...and it's not even hard at all. it just requires a lot of editing of his library for the most part.
If you know vJASS, you can just use the execution order of libraries to get what you want. Just create the random creeps with a library's initializer, then make this library require the library in question.
Other than that, there's no way. Units that enter the map after map init will be ignored, no matter how they entered.
actually i found another way...and it's not even hard at all. it just requires a lot of editing of his library for the most part.