Convert point XY to page units XY using arcpy?

I am using arcpy to export PDFs from my ArcGIS Server application. I want to move a TextElement in my layout view to the centroid of a selected feature. I can get the centroid of the feature, but that's in my projected coordinate system. I need to convert that point into inches in my layout view.

elements = arcpy.mapping.ListLayoutElements(mapDoc, "TEXT_ELEMENT")

    for elem in elements:
        if elem.name == "SiteText":
            elem.elementPositionX = center.X
            elem.elementPositionY = center.Y

center.X and center.Y are in a projected coordinate system, not inches. I know it can be done with ArcObjects, as I've done it in desktop addins using C#. I need to do it from arcpy and I don't see how.

Answer

You can use the data frame extent to scale the projected coordinates to map coordinates. I believe this should work well for rectangular coordinate systems. The returned map coordinates can be used to update your text element positions

def proj2map(data_frame,proj_x,proj_y):
    """Convert projected coordinates to map coordinates"""

    # This code relies on the data_frame specified having
    # its anchor point at lower left

    #get the data frame dimensions in map units
    df_map_w = data_frame.elementWidth
    df_map_h = data_frame.elementHeight
    df_map_x = data_frame.elementPositionX
    df_map_y = data_frame.elementPositionY

    #get the data frame projected coordinates

    df_min_x = data_frame.extent.XMin
    df_min_y = data_frame.extent.YMin
    df_max_x = data_frame.extent.XMax
    df_max_y = data_frame.extent.YMax
    df_proj_w = data_frame.extent.width
    df_proj_h = data_frame.extent.height

    #ensure the coordinates are in the dataframe
    if proj_x < df_min_x or proj_x > df_max_x:
        raise ValueError ('X coordinate is not within the data frame: %.1f - (%.1f, %.1f)' % (proj_x,df_min_x,df_max_x))


    if proj_y < df_min_y or proj_y > df_max_y:
        raise ValueError ('Y coordinate is not within the data frame: %.1f - (%.1f, %.1f)' % (proj_y,df_min_y,df_max_y))

    #scale the projected coordinates to map units from the lower left of the data frame
    map_x = (proj_x - df_min_x) / df_proj_w * df_map_w + df_map_x
    map_y = (proj_y - df_min_y) / df_proj_h * df_map_h + df_map_y

    return map_x,map_y