It is possible to label multipoints such that the centroid of the set of points for a multipoint is labeled. The function gs:Centroid is used the SLD. The centroids are calculated on the fly and can be symbolized using the same attributes as the original points.
In this example of points it can be seen that if a multipoint has only one point, the centroid is on top. If the multipoint has two points, the centroid is in between the two points.