It is very possible that the child Miriam is a figment of Mrs. Miller's imagination: Capote's own interpretation of the story is that she is. (An author's own interpretation of a story is not the only one.)
If Capote's interpretation is true, then perhaps the brooch is still sitting in Mrs. Miller's jewelry box. Or even more likely, Mrs. Miller herself is the "Miriam" who wears it.
The mysterious child seems to express Mrs. Miller's own repressed desires. Mrs. Miller is described as wearing clothes that
were matter-of-fact, her hair iron-gray, clipped and casually waved;...her features were plain and inconspicuous.
In contrast, Miriam has flamboyantly long, wavy white hair and wears an elegant plum velvet coat. She is anything but inconspicuous. Later she wears a striking white silk dress with a pleated skirt. When she demands the brooch, it is very possible she is enacting Mrs. Miller's own repressed desire to wear this beautiful piece of jewelry. If Mrs. Miller is both herself and the young Miriam, then it is she who has put on the brooch.
Miriam appears to goad Mrs. Miller to do what she wants to do but won't allow herself to do. Mrs. Miller probably wanted to get rid of the paper rose and glass vase that "Miriam" broke and to replace it with what she buys: a vase she calls "grotesquely vulgar" and six real white roses. Mrs. Miller seems to be able to only do what she really wants when she pretends that she is doing it for Miriam and when she divorces her conscious self from responsibility for the act. For example, Mrs. Miller buys the vase
as if by prearranged plan: a plan of which she had not the least knowledge or control.
Mrs. Miller gets the new vase, the roses, the candy, and the cake, all of which she wants, as if for the child. Therefore, it is likely that she put on the brooch but managed to convince herself that the child took it. Of course, this is an informed supposition: the story never literally tells us that Mrs. Miller wore it.