A www/email to fax gateway is straightforward; a week's programming at most.
The problem is that there is no real confirmation of delivery, which you get from a direct fax which you send yourself. PC Plod won't give a flying **** if you show him an email confirming that the fax was actually sent...
This is why I send out the GAR from a laptop with winfax on it, and a GSM/GPRS card. It's a pain, but when it's sent you know it's sent. A PC fax like this is also dead handy for faxing foreign airports for PPR etc; many or most airports in Europe are actually PPR and the further south you go the worse it gets. Many are 5 days PPR, some longer.