From the PHP Manual section on Language Options:
If your application can use PHP4, you can simply set your domain to use PHP4 (from within the DreamHost control panel) and the Allow_call_time_pass_reference option will be set to "ON".
That said, and to answer your actual question, there are two ways on DreamHost for you to edit php.ini to change certain lanuage options:
http://wiki.dreamhost.com/Installing_PHP5 / http://wiki.dreamhost.com/Installing_PHP4.
You should note that neither of these methods is supported by DreamHost Technical Support staff; if you go this route you are "on your own".