Does anybody know format of .dat file to help with bookmark automation?
I agree with @PolyGeo you will need ArcObjects to process the BookMarks. In fact it is not a straight forward just get the extent coordinates as there are 2 types of Bookmarks and you would deal with them differently: AOI and Feature bookmarks.
In fact if anyone can prove me wrong I don't think there is even an Interface that allows you to connect to an ArcGIS Place File (.dat) and read/write directly from it? Nor does it seem you are able to load existing Place Files into a Map Document. I think it's one of those weird situations where there is a dialog exposed by the ArcMap application but there is no way to hook into it.