OK, here is what I think is correct.
The beginDwgOpen() reactor sort of matches up with DocumentCreateStarted event handler in .Net. This is a misnomer since it is fired when a document is created OR an existing document is opened.
The event arg filename property will be String.Empty if it is a new document otherwise it will contain the filename of the existing document being opened.
If anyone knows of a different way to do it please let me know. I'm really just guessing at this point.