Searchable key words (tags) for podcasts

The ID3 metadata system is very small, and is tagged onto the end of a typical MP3 file. Nowhere in the specification does it mention key words, and that will be why the field doesn't exist.

There isn't a format available to save in in Audition that is actually suitable for talking books, and this is pretty general - which is why you'll find that these use proprietary formats specifically for the application. If you want to start putting key words, tags, or anything else like that into podcasts, then it looks as though you'll have to use one of the talking book formats - which means that your readership will be limited.

All of the basic audio formats were designed, along with their metadata, well before podcasts were invented. Updating metadata formats can run you into compatibility difficulties, which is generally why it hasn't been done. ID3 has been updated in its lifetime, but the updates don't include anything to do with key words. I think that this is because there simply isn't room in what is apparently a pretty small MP3 data field that was primarily intended for  track identification files at its inception.

There is quite a lot of information about what you can identify in ID3 here.