Dirk, another thing I've learned in all my years of gunsmithing is that taps DO wear out! When they start getting dull you're inviting disaster by continuing to use them. I'm not sure how hard aluminum is on a tap or how many times you used that particular tap so I'm not passing judgement on your actions, just stating what I've learned. When I built my son's STI 2011 I used a slide that already had the Bomar sight cut from the factory, but for some strange reason they don't tap the elevation screw hole! STI slides have a reputation for being some hard steel I found out later, AFTER I broke the tap off in the slide! The tap had been used maybe 5 or 6 times before, and yes, I was using oil. Fortunately my machinist friend in El Paso is also an excellent welder and after a phone call and an afternoon drive to El Paso the problem was solved. The guy literally welded up a nub on the very small part of the tap that was protruding and then just took a pair of pliers and turned the tap out! I ordered a new tap from Brownells and finished the job. I could definitely feel the difference between the old tap and the new one when I finished tapping that hole.
BTW, cool solution you came up with! I definitely have to remember it in case I have a similar problem. Taps are evil! Just when you think things are going great they will mess your day up!