Not that i expect anyone would ever do this... but could someone use a shrunken head for the l'rosh v'lo l'zanav siman on Rosh Hashanah?
Basically, what kind of a head are you talking about. If you mean a human shrunken head, then it is subject to all the rules of a body of a human being. This includes the requirement that it be buried, treated with respect and the laws of tum'ah (ritual impurity). As such, using it as a "segulah" would be asur.
In fact, it is also asur to have a (real) one in your house as you should bury it as soon as you get it.
The differences in treatment between the head of a Jew and a nonJew are the same as that of any dead body.
If you are talking about a "shrunken" animal head (such as a sheep) there should be no question. In the case of an animal head, you should make sure that it is from a kosher shechted animal so that you can eat from it.
The gemara in Krisos daf vav says we eat the simanim. The girsa in Horios 12a is to look at the simanim. In situations where one cannot eat the siman, we rely on simply looking at them. This was the Ben Ish Chai's practice when a certain one of the simanim was too bug infested to be allowed to be eaten.
Being that the link was to a human shrunken head I will only adress that. See the Yad Ephraim printed in the standard Shulchan Aruch Yoreh Deah in the begining of Siman 117 where he rules that mummies, which were thought to have medicinal properties, are assur bihanaah. See also the end of siman 142 where the Shulchan Aruch rules that looking at something for pleasure is called getting hanaah, and although the Shulchan Aruch is discussing avoda zara, the Shach there tells us that other issurim are even more simple to us as being assur. Assuming that the looking at the item for a good siman is hanaah, it would be assur to use the shrunken human head.
And besides, ew.