I lost the bolt once, use a allen wrench and tape to get me through a bicycle event where I was safety cycle. When I got bolt (2 actuality) I use loctite and never had issue. I check it once in a while.
With the footpeg mount unmodified, the pivot bolt barely protrudes out but not all the way 'round. If you drilled the threaded end of bolt for safety wire, you'd still have to grind away some of the footpeg mount to even see the hole.
Oh then it be best just to do it the way you did it.
My first idea was to drill it and use a small cotter pin & flat washer but I ground more clearance so I could use a nut.