I totally disagree with these guys. If you want a showroom stock restoration, then they are right, but you could easily get a humbler rear derailleur which clamped on.

They are usually used with bolt-on rear wheels but all you would need is a new axle and your bro's old track nuts to convert your wheel to bolt-on. I don't know if they require a solid axle, though, because there are models with a small retaining screw which fits into a nut which is shaped like the back of the dropout and retains the derailleur in position while letting the axle handle the clamping loads.

There are also cable clips.

I say put up with a few grams more weight, get a more basic rear derailleur, snap on the cable clamps and get some clamp-on shift levers, and bike away. Easy as pie.