How can I modify the media upload form fields?

Would maybe look into do_action( 'pre-upload-ui' );.

Here is my first thoughts, please note it’s not tested but should get you in the right direction.

add_action( 'pre-plupload-upload-ui', 'sd_extra_info' );

function sd_extra_info()
{
    # see wp-includes/media-template.php
    print '<pre> Some more info here </pre>';
}