Customize the 'add new product' interface in marketpress

Hi
I want to customize the 'create new product' screen. I've used the remove_meta_box hook to successfully remove the author meta box, slug meta box, etc. However, its not working on marketpress specific meta boxes. This is the code I've used to try and remove the product download box:

function custom_meta_box_removal() {
	remove_meta_box('mp-meta-download', 'product', 'normal');
}
add_action('add_meta_box', 'custom_meta_box_removal');

Is there anyway to do this without deleting it directly from marketpress.php?

Also, I'd like to take out the external link field from the product detail meta box - can that be done w/out deleting it directly from marketpress.php?

Many thanks
Andy