Dynamic overlay: how ?

Hmmmm...Perhaps its easier to just get original action file from Imagecache Actions and patch it a bit

It is easy. Just use #325103: Custom action support - first step to dynamic actions and original code from imagecache actions module:

// Calculate $x, $y, $alpha
$overlay = imageapi_image_open('path_to_file.gif'); 
return imageapi_image_overlay($customaction_image, $overlay, $x, $y, $alpha );