If you can get a hold of some modem software/firmware source code that can be retrofitted to work on audio signals from the wave audio feed of your choice instead of the hardware it was designed to talk to, I will consider this remarkable in and of itself.  Most of those are kept as proprietary as possible.

You won't get 56kbps by imitating a modem, since you can't get 56kbps peer-to-peer even with a real modem.  Achieving 56kbps is done by very carefully exploiting properties of the telephone lines they'd be used on.  For starters, achieving 56kbps is possible only under the most ideal of conditions, and in one direction only: from the ISP to the customer, and the ISP must have special head-end equipment that outputs the audio data in digital form directly onto a T1 or ISDN or similar line in order for it to even be possible, so as to cut out the loss going from analog to digital.

The modems themselves can push no greater than 33.6kbps to another modem, no matter how pure the connection.  The protocol simply doesn't allow for it.  And this is in the best case.

The attenuating cable you've mentioned as an example would be a nightmare.  There's no way to know how much attenuation is the right amount.  And just by looking at the cable, I can see a clear problem: it's meant for mono voice recording.  Rather than having a 3-point connector (ground, right audio, left audio), it has a two-point connector (ground and signal).  Plug this baby into a stereo jack, and you literally short the right channel to ground (compare it with a normal headphone jack and you'll see why - the right channel is the middle connection point, and the ground sleeve is shorter).  Your typical user is never going to know this without being taught.

Wasn't the original purpose of this idea to make it convenient for the user?  If the user's got to go to Radio Shack to get an unusual cable to do this communication, then he ought to just pick up a null-modem serial cable, and know with 100% certainty it's going to work.