matt,
you should be able to change any php.ini directive using a .htaccess file formatted as follows:
<IfModule mod_php4.c>
php_upload_tmp_dir <directory>
</IfModule>
i'm currently using this to turn off magic quotes with the following code for php3...
<IfModule mod_php3.c>
php3_magic_quotes_gpc Off
</IfModule>
hope it helps!
-stephen