Allow dynamic header height in a theme based on twentyeleven

Hi, I have been trying to allow the bloggers to upload headers with a fixed width but a dynamic height.
No luck so far. Have tested this plugin but it don't play nice with multi-site:

I found this page that might be of help but since I lack php knowledge I'm not sure how to accomplish it fully:

What I'd like is to have something like this in functions.php

define( 'HEADER_IMAGE_WIDTH', apply_filters( 'twentyeleven_header_image_width', 900 ) );
define( 'HEADER_IMAGE_HEIGHT', apply_filters( 'twentyeleven_header_image_height',  $dynamic ) );

along with some filter hook to allow for dynamic header height when uploading a custom header.

Is it even possible?
Many thanks