Little update here. First, I replaced my cracked headtube. You can see it all
here in this thread.
Here are some highlights.
SECONDLY, I recalled a ride in which I accidentally kicked my monster front derailleur while trying to clip in and pedal uphill to keep my buddy from passing me. It went badly as you might imagine. The derailleur was severely askew and locked up the cranks. I bent it back and continued the ride and everything was fine except for the crack in the fork and the head tube!
Try doing that with Di2!
I had made the front derailleur without a little reinforcing diamond on the seat tube. Later Herse bikes have this feature, but my '46-'47 tandem does not. I figured it must not really be necessary. Well, I think now I know why Herse started putting it on! It was no accident or frivolous flourish, but a real necessary bit.
So as long as I had the frame in the shop, I spent 2 hours and made one and put it on.
It is cut from the (oversize) downtube of an old crashed Rockhopper, run into by a snowplow while parked. The top tube was curved like a banana. I humanely euthanized it and now it's a metal donor. Parts of it have gone into a Jack Taylor reproduction stem, as well as some other repairs and things! All you gotta do is draw the shape you want on the tubing, put on your safety glasses and ear protection, and carve away!
I made it in two pieces and brazed it on with black flux and Harris Safety-Silv 56. The idea being that if I braze it on in two pieces and am careful with heat, I won't disturb the front derailleur positioning, which is as good as I'll ever get it.
You can see the split here.
Hopefully this shores up the front derailleur enough to take the strain of an errant foot next time I fumble an uphill clip-in!