Well, another site I have posted on appears to have come up with PSK31 interface which utilises the soundcard microphone input.
With PSK31, the operator typically uses a single sideband transceiver connected to the sound card of a PC. When the operator enters a message for transmission, the software produces an audio tone, which sounds to the human ear like a continuous whistle with a slight warble. ***This is then fed through the microphone jack and an intermediate resistor used to reduce the sound card's output power to microphone level at the transceiver (or an auxiliary connection is made to it). The diagram for the module shows just the tip and sleeve of the plug being used, so I will try that as it looks the most likely solution.
I completely agree with what you say, though. It has been incredibly difficult to find out about the mic input requirements, which call for a mic of 200 - 600 Ohms impedance to be used.