Not sure if this has appeared here earlier, but I think various people had in the past proposed knocking up a website where one would enter the details of the flight, and it would advise the correct authorities.
Actually a while ago I spent a few hours writing just that. An HTML form/CGI script combo (plain HTML so usable from your iPad/Android as well) which would ask the required information, use the ICAO identifiers of the airfields involved to figure out where to send the info to, and fill in the (official) GAR PDF with the required information.
It would then e-mail the GAR form to the proper authorities, with CC or BCC to the submitter. (And would notify you of any authorities that would not accept e-mail, so you can fax the form yourself to those.)
I abandoned the project at the stage where I would need to fill (and maintain) the database of e-mail addresses, and where I had to implement a PayPal "volunteer donation" scheme to make a little money to pay for the development/maintenance time, server and bandwidth. (I figured that if 10% of the users would donate 1 UKP per GAR form sent, I would already break even.)
The negotiation part sounds a tough one but the reality is most agencies already accept GARs via email, usually leaving just the one agency to be convinced. We did this in Dorset by replicating the content of the official form visually but in particular ensuring the wording was exact, that made them happy.
My script uses perl and the PDF::API2 to take an official empty PDF GAR form, and place the data fields exactly where they need to go. So the agencies receive an actual PDF GAR form, not an almost-like-a-GAR-form-email. And that PDF is obviously sent as an attachment.
I'd be happy to donate the code that I wrote for this to you if you're interested, Nibbler. It might make your negotiations easier.
(Edited: PPRuNe at it again. This was submitted at 11:00, *after* the post of IO540 which appears below.)