It is certainly possible to fax e.g. from a laptop with a GSM modem (e.g. a mobile phone with a fax-transmit-capable SIM card, most PAYT SIMs don't support fax) attached to it with a cable, IR or B/tooth.
On e.g. Navbox (
www.navbox.nl) you can go click-click-click to generate the route, and then it generates the whole ICAO FP, which you can fax straight out with a PC fax program e.g. Winfax. Very neat.
But one needs to carry a laptop, whose batteries are likely to be flat when you really need it.
One could fax out of a GSM-capable PDA and
AFAIK one can run Navbox on a Pocket/PC PDA. That's probably the most portable way of filing flight plans right now.
BUT a website would be great, especially ............ as there could be software behind it which would tell you if you are going to go into CAS, breach danger areas, etc!! Could also check Notams. No, that would be too good so it can't be done
The Bristol thing has gone, I last heard.