Hi Bazza and thank you.
I too enjoy finding simple (and sometimes unusual) ways to solving a problem.
In this case the lack of "traditional" I/O ports on modern computers.
As regards your mono mic input dilemma, have you or are you willing to consider using those small, cheap USB sound cards?
They sell for around 3 or 4 Pounds in some shops and even cheaper online.
They also have only mono mic inputs, but using two of them (or one of them and the internal sound cards mic input) and splitting the audio input to both mic inputs might help you.
Those USB sound cards comply to the standard USB Audio spec so no drivers needed (at least in Linux, don't know about the Mac) and show up as /dev/dsp0, /dev/dsp1 and so on.
BTW, connected one to my Android based phone via a micro USB to "normal" USB adaptor, Android picked it up immediately and I could use it.
Here is a pic of one of them (plenty others around):