Since we don't have access to php.ini, and the php_value and php_flag directives are disabled, you're limited to ini_get() and ini_set(). You'll have to manually include a configuration script, since there's no way of setting auto_prepend_file. Sadly, that won't help with some settings, such as magic_quotes_gpc. For that one, you'll either need to test get_magic_quotes_gpc() to conditionally run stripslashes() when you access user input, or have your configuration script do it, which is easier but potentially wasteful.
Code:
if (get_magic_quotes_gpc()) {
$_REQUEST; # so $GLOBALS['_REQUEST'] exists
foreach (array('_GET', '_POST', '_COOKIE', '_REQUEST') as $k) {
$GLOBALS[$k] = array_map('stripslashes', $GLOBALS[$k]);
}
}