Batch processing would have to be based on the categories currently present in combination with some sort of natural language processing to pick up keywords. That could lead to some interesting metadata creation.

Perhaps another tack may help you find greater satisfaction. Q. Do all 680 some messages need category makeovers? If so breaking the task into stages might make it manageable for a human being. Given your tree of categories, is there one category in particular that in your opinion requires work? Start there. Do one entry a day. Or limit yourself to a one hour session per week. Report back on the blog in a highly pomo self reflexive fashion on the results. Move on to the next category. 680 is a finite number.

I suspect you are also considering the backlog that might accumulate if you don’t use your category tree on entries you create henceforth. This entry, for example, “Category Mistake” mihgt fit under “to do” which is nicely empty at the moment or may it belongs under “uncategorizable” which provides a lovely set of paradoxes: labeled, it is categorized.

Of course some of your readers could volunteer to read through your entries, with an eye on the category tree you have developed, and make suggestions. Your blog could be adopted by an information sciences class.