Maya Python: worldspace to screenspace coordinates
I finally found how to do this on the internet. Just call this procedure with a camera name and the worldspace coordinate of a point as attributes.

def worldSpaceToScreenSpace(camera, worldPoint):

    # get current resolution
    resWidth = mc.getAttr('defaultResolution.width')
    resHeight = mc.getAttr('defaultResolution.height')

    # get the dagPath to the camera shape node to get the world inverse matrix
    selList = om.MSelectionList()
    selList.add(camera)
    dagPath = om.MDagPath()
    selList.getDagPath(0,dagPath)
    dagPath.extendToShape()
    camInvMtx = dagPath.inclusiveMatrix().inverse()

    # use a camera function set to get projection matrix, convert the MFloatMatrix 
    # into a MMatrix for multiplication compatibility
    fnCam = om.MFnCamera(dagPath)
    mFloatMtx = fnCam.projectionMatrix()
    projMtx = om.MMatrix(mFloatMtx.matrix)

    # multiply all together and do the normalisation
    mPoint = om.MPoint(worldPoint[0],worldPoint[1],worldPoint[2]) * camInvMtx * projMtx;
    x = (mPoint[0] / mPoint[3] / 2 + .5) * resWidth
    y = (mPoint[1] / mPoint[3] / 2 + .5) * resHeight

    return [x,y]