The mod already supports the searching original en_us text (which the upside down variant is directly based on), so I don't understand why should the mod add special logic for this. The original post might've been posted earlier than this feature was added though.

Can you elaborate? Do you want to be able to search in the creative and recipe book menus in any of selected languages (including English, US)?

search the english equivalent text and right side up
English finds ɥsᴉꞁᵷuƎ
Upside Down finds uʍoᗡ ǝpᴉsd∩