As a last resort get a mechanics telescoping magnet and stik that down into all the crevaces you can get to. You do the old trick of in pitch black get you a small very bright light and start flashing it down into every thing. I assume you did start the bike up and rev it up to see if the nut would fall out. Keep looking and maybe switch sides to look for it.
Fossil pointed out a handy pickup tool that is a good idea to have in your tool box. :sThat area where that nut could fall into is a very tough area to access, it picks up road junk and gravel all the time.
Find myself constantly using engine brite and water to clean that area and stuff rag with popsicle sticks and such to try to get in there.:newsmile077: