If the loosening is due to the fibre washer crushing, a locking wire would not help, and in fact, would be a PITA.
I decided losing a bolt or two was more of a pain, and it's not wired but just a locking clip, and all flats are drilled so all options covered to be a *fairly* straight push through and lock....
I've said before that being unable to easily source fibre washers I made my own from 1/16" nitrile sheet and slightly oversize OD and the holes snug on the straight shaft bit before the taper- seems to work for me and haven't touched it in a few thou miles
There was a discussion soon after that about the dowty washers and I think the size 11/16" comes to mind- maybe BSP
which didn't sound right to me, but was no further debate....