[Nursie]

> If someone was to build a device that could clamp on to the phone line and listen for incoming faxes and copy the signal and sent it of to a remote server,

Pretty sure that fax-to-email gateways have existed at least since the early 90s, most consumer-grade modems could send and receive faxes (with software to save them as images). Hooking that up to an email sender seems trivial.

Hell, back in '96 an old friend needed to get a signed letter to the bank by close of business one day, so he used a tablet to sign a document and then the modem to 'fax' it over.

--edit--

I misunderstood, you were talking about a MITM attack to prove the insecurity of faxes. Ignore me, carry on :)