Was looking at this too .
raids_form.tpl is the raid definition by the looks of it an a line could be added there
I "think" the checking is done in includes\functions.raid.php
// displays the popup signup form
function signupForm($data, $pMain, $redirect = null, $showComment = true) {
global $pLang, $pConfig, $db_raid;
$sql['SELECT'] = '*';
$sql['FROM'] = 'character as c';
$sql['WHERE'] = "c.char_level >= {$data['minimum_level']}
AND c.char_level <= {$data['maximum_level']}
AND c.profile_id = ".$pMain->getProfileID();
$sql['SORT'] = 'c.char_name';
$result_chars = $db_raid->set_query('select', $sql, __FILE__, __LINE__);
I will have a poke around and see if i can get it work then Ill pop it up on the hydra calendar and if that works ill do the new alliance one an send you the files -- or Ill hope that someone with more brains with me either does it

or points me in the right direction.
As an asside it may well be worth adding this as a feature as a lot of games now need a certain attribute to be able to access certain areas - LOTRO is nice and easy as the only real requirement is a Radiance.