Yes, I could just retap the braze-ons to 5 x 0.8 mm, since I will never install Cyclo shifters, and probably nobody else will either. But I don't have a "bottoming tap" for that, and am loath to drill through the downtube just to retap....
...these small 5 x 0.8 taps are cheaply and readily available at places like Home Depot (at least my HD), and on Amazon.
It's not a big deal to buy an extra one and use a grinder to take down the tapered end. ( Actually, they are so hardened and brittle that it's often not hard to just snap off the tapered end.)
What you get as a result is a passable bottoming tap. You start the re-thread with a plug tap (slight taper), and when you run out of room you switch to the modified one.
In case all your workarounds end up going astray, this is probably a better alternative than removing the braze on. (I think....maybe not.)