Hi Craig,
The custom keywords meta box is only available for custom post types, not for taxonomy terms (at least not yet). I am not sure if this would work, but if you can somehow add term meta fields to the taxonomy, then you could use it to store this information as well (similarly as the post meta box).
ACF plugin is capable of creating term meta fields, which then you can use on the taxonomy editor.
Then, on the plugin back-end, you can enable the search in term meta option, and this field will be used, when looking for taxonomy term results.
I hope this helps!