For anyone interested, here's how I made the repair:
1. Filed down the remains of the broken lug until it was flush with the breechface.
2. Made a centerpunch mark that permitted an 1/8" hole to be drilled that cleared the bottom and side edges of the corner by approximately 1/32".
3. Drilled the 1/8" hole using a 6" long cobalt aircraft extension drill bit to a depth of 5/32".
4. To make the replacement lug insert, I used the shank end of the drill bit and filed a 'flat' 1/8" in height. I continued to file down the flat until it could be inserted into the hole while just clearing the rim of a cartridge. (To check for fit, I sawed off the rim of a case and set it on the breachface.)
5. Once the flat was completed I cut off the piece from the drill bit taking care to make the shank slightly longer than the actual hole depth. I inserted the insert into the hole, checked for fit, and filed as necessary to make the final fitting adjustments. When done, the lug was even in height to the adjacent existing lug and the flat at the bottom of the lug was flush with the breechface.
6. In order to sufficiently clear the rim of the cartridge, the lug insert had to be filed down to where there was only a small segment of metal remaining. I seriously doubted that this small segment would have been able to withstand the repeated hammering and movement of the cartridge base during the cycling process. To resolve this I filed down the corner of the slide to increase the surface area around the insert and then welded the bottom section of the insert in place.
To reinforce the upper section of the insert, I built up the remainder of the insert by brazing it and then filed it as necessary to replicate the profile of the original lug thus creating a bronze overlay over a steel core that is now capable of handling the cycling process. 
7. The heat from welding and brazing did cause some distortion to the slide that required fitting and lapping in order to mount it back onto the frame.
Since the repair, I have fired fifty rounds through the pistol without any problems. The pistol now functions flawlessly.
The overall condition of this particular pistol appeared as though it was shot quite a bit and not very well treated during its service life. (I have another P-64 in much nicer condition that has never had any problems.) However, I don't think that lack of care would have caused the lug failure. The lug was intact when I purchased the pistol (it might have had a hidden crack that I never noticed?) and I had fired more than three hundred Silver Bear FMJ cartidges from it before the failure occurred. I did experience a few FTF jams but never any FTE issues.
My best guess as to the cause of the failure is that the old / weak magazine springs prevented the last one or two cartridges from moving up fast enough. As the slide cycled forward, the cartridge was too slow to move up properly and subsequently was caught on the side of the case and shoved forward on an angle (similar to a 'stovepipe' jam). When the cartridge moved forward in the canted upward position it was wedged between the feed ramp and the two lugs with the full force of the slide pushing against it. The slide's energy was being directed against the inside tips of the lugs, essentially attempting to force them apart. I believe that is what may have caused the weaker of the two lugs to break off.
(BTW, the weak magazine springs prompted me to find a way to make my own replacements. Here is a link to my post describing how I made them viewtopic.php?f=3&t=4692 .)
Anyone having a better theory as to why or how this failure may have occurred please chime in.
HD77
