That sounds a lot like vintage rear dropouts are installed. Brazed in place, then any gaps filled with brass (I think).

With a lathe, one could make similar tabs out of 3/8" round bar stock (OD of your tubing). cut one end down to the ID of your tubing. Leave a shoulder for support, and then grind, forge, or machine the other end to your suiting. Drill the hole and braze together.

I assume most modern racks simply have the tubing ends smashed together. You could do it with a vice. Then using brass to fill the gap and drill. For thinner tubing, you could sandwich in a third piece, although getting sizes perfect would be a hassle.

I have an old Burley rack and most of the tooling for making them.

The Burley rack used a 3 tube support for the rack, with the middle tube larger than the others. The bottom of the middle tube was cut at a 45 degree (or so) angle. Then a plate with a 135 degree (or so) bend was welded or brazed to the end of the tube. The plate had 2 holes for adjustment, and a slot to catch the pannier spring/bungee.

I'll try to get a photo shortly.