Gibbet -
Have you tried heating the brass and bending? An alternate might also be bronze cotter pins, their already preformed. A lot of buckle pins I've seen are steel cotter pins with the bottom trimmed.
A number of years ago I bought two semi-finished buckles, one brass and one german silver both came with cast pins that had to be
finished and bent into place.
Perhaps in future castings a cast pin could be made into the design and cast separate but at the same time as the buckle sort of like the plastic tree on a model airplane, etc. parts.
Jas. Hook