Thanks, @herkules:
I took what you did and amended slightly. This version offloads the domain replacing part to the MU Domain Mapping plugin:
add_filter('wp_get_attachment_image_src', 'acf_multisite_url_fix', 10, 4);
function acf_multisite_url_fix($image, $attachment_id, $size, $icon){
if( isset($image[0]) ){
$image[0] = domain_mapping_post_content( $image[0] );
}
return $image;
}