Friend/jtpcopp: First, friends share names. What is your name?
Second: Marty used a bolt to plug the unsightly hole in the derailleur mounting bracket, and further, chose a bolt that more or less matches the allen axle bolts.
Third, I think it's a good plug, but I've gone to extremes on both a custom Riv Road (but one less good than my later ones) and a very nice pre-susp DB mtb by cutting, Dremeling, hacking, grinding, filing, yanking, bending, twisting, pulling, and otherwise removing and destroying the derailleur hanger when I converted the frames to dedicated, never-again-reverted fixed gear use, and I must say, tho' I say it myself, that the results were impressively good.
Patrick Moore, who just had a very nice detoured grocery store ride in 70*F ABQ, NM on the non-Riv clone of a former Riv custom.