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I always up to SS first for spelling, re-edit and then send to others

Mozilla firefox has spell checker also...

I was thinking along the same lines as dirkr except using Lightroom which has a built in facility for importing and exporting the master keyword list which you could spell check. If you check the keyword list you would not need to run a script against individual files.

If you spell check the master list, any spelling corrections you make in Lightroom would be automatically applied to all the files that have the misspelled keyword if you correct it this way: find the keyword in the master list (not in the image file), select it, right click on it and choose 'edit keyword' then fix the spelling in the pop-up box and click on 'save'. Of course you'd need to do that before you export the file.

I know that doesn't exactly answer your question but Bridge (which I don't use) probably has some very similar keywording features you could use to accomplish the same thing.