The most simple solution is "hard code" the new settings in member.php.
Find:
include(ROOT_PATH.'includes/page_header.php');
Insert above:
if (in_array($cat_id, array(1,23,4,5,10)))
{
$config['max_image_width'] = 800; //image width
$config['max_image_height'] = 800; //image height
$config['max_media_size'] = 1000; //max media filesize in kb
$config['max_thumb_width'] = 100; //thumbnail width
$config['max_thumb_height'] = 100; //thumbnail height
$config['max_thumb_size'] = 50; //max thumbnail filesize in kb
}
numbers in first line "1,23,4,5,10" are comma separated category IDs;