You can hook into the jpeg_quality filter and do a check for the post type you want to apply the filter to.
add_filter( 'jpeg_quality', 'my_jpeq_quality' );
function my_jpeg_quality( $quality ) {
if ((isset($_GET['post_type']) && $_GET['post_type'] == 'your_post_type') || (isset($post_type) && $post_type == 'your_post_type')) :
return 100; //Set quality here
endif;
}