One can buy brass hex head nipples.
For example Sapim Brand which does also come in black.
https://www.jejamescycles.com/sapim-...r-tmm8539.html
Then it would be a matter of finding a socket or nut driver that would fit into the spoke holes and fit the nipples.
Some brands also have a square drive, or reverse torx drive which might play better with the spoke holes in double walled rims.
