Technically speaking you could just do a cr.execute() call to get exactly what you want. That's what cr is in nearly every python function, the database cursor. There's no XMLRPC command for joins though. Check the psycopg2 documentation for more info on how to use that properly. http://initd.org/psycopg/docs/usage.html

cr.execute("""SELECT account_invoice_line.id
    FROM account_invoice_line LEFT JOIN (
    SELECT id FROM account_invoice WHERE type='out_invoice'
    ) AS foo ON foo.id=order_line_id
    WHERE product_id=%s""", str(my_product_id))
results = cr.fetchall()
invoice_line_ids = []
for res_id in results:
    invoice_line_ids.append(res_id[0])

Note that the result will be a list of lists of a single integer, so that's why I included a for loop to convert it to a single list of IDs. Also, converting the integer to a string first is proper psycopg2 syntax.