It is funny, Rebecca, that you brought this all up for discussion. I have been interested in miniature portraits for a while now, I guess in part because I succumb to various (art-related) challenges. So, recently I ordered a set of miniature brushes and a little ivorine. The ivorine is a substitute that they use today as ivory is illegal. Sometimes you can find old piano keys that are being offered for sale and they were ivory.
I have not tried the actual tiny painting on ivorine yet as I have been searching for old watch fobs in order to do exactly as Marvin mentioned. I did not want to cut the ivorine to size until I knew the size. I am still searching for watch fobs (old or new) and lockets or pins. If anyone knows of such an outlet, let me know. I have one "practice" piece I tried on a piece of board. I will try to post it soon. It is not a good one yet, as I see I need to consider more practice. It was a 2 1/2 inch by 3 1/4 size. I do enjoy it.
I found the history of them being quite like that information that Michael gave here. They were certainly popular years ago due to lack of cameras. But I am not sure of the saleability of them.
I want to see what you have done already.
Ditto on the fact they are tedious and yes, I do think that one should get a good penny for them due to that fact and the fact that there are not that many to be found today.
Good Luck.