You can enamel on brass. Art Enameling on Metals by H.H. Cunynghame refers to enameling on brass. And Thompson’s Enamel sells it. Gilder’s Metal is how it is listed and it is copper with a bit of zinc, 95% – 5 %. It works with transparent enamels, sold for gold, silver, and copper, = medium expansion enamels, according to the their experts and you have no need for flux as it does not oxidize like copper.
Don’t fire it too often. After three or more firings, the enamel can jump off. Enamelled badges and emblems are very often made of gilding metal.