Now that my touring frame is done another non bike project is on the agenda. I want to fabricate a slide bolt setup for a door using 1/2" copper pipe. Is it feasible to brass braze mitres on copper?

Sure, but I have to ask what purpose the copper serves? It's awfully soft for much of anything structural in thin sections.

If you're fixated on using copper I think a safer choice – mostly due to the heat of brazing – would be silver soldering between the two metals.

You have a plan showing details of what you want to fabricate? That may help those of us out here understand what you want to achieve.

That's a lot more like welding than brazing, although brass does melt at 200 degrees less than copper. Blowouts are pretty likely. Do it, it will be good practice. But HVAC technicians use silver for a reason.

I have brass welded together a brass violin plane. It's frustrating because of the blowouts

For my third frame build I made a small rack attached to the upper seat stays. Think of a TA front rack that bolts to a Mafac brake but brazed to the frame and behind/under the seat. The rack was made from K&S brass square tubing (use what you know and can get, there was a hobby shop across the street and I did a little slot car stuff as a youth. But what did I know about metals back then would fill a book). I used 56% silver for the rack joints and the SS attachment. Even with the lower temp silver I saw some sagging of the square shape here and there as I got the brass square tubing too hot.

The rack worked well enough for my small tool kit, the spare sew up toe clip strapped to the seat rails. Until it didn't and got bent askew during a crash. I have only attached a rack directly to a frame once again since. That time the rack was round steel tubing. Andy.

K&S brass reminds me that State College can't keep a hobby shop in business. Closest we got is a craft store. I guess the internet killed hobby shops. Ace hardware might still have an assortment from K&S though.