As someone who these days is more involved with audio\visual than aircraft I suggest a simple splitter.
If your headset connector is just a standard jack or XLR type (most are B gauge jacks or similar I recall...) then it's pretty easy to buy some jacks & sockets, with a bit of cable & with a bit of a steady hand, some solder etc, split the RT audio output at the jack into a tape\minidisc\memo recorder\anything else that will record. The soldering job is a max of 20 minutes work.
In theory you wouldnt even need to connect the TX side up for the recording device as you wouldnt be using it to transmit - one hopes!
Given you're not modifying the RT system & headsets are unregulated, so long as you wire things right & make sure the quality of audio is unaffected (no reason why it should be!) then you have yourself a splitter.
Try Maplins for the bits...