I wanted to use a standard computer headset and footswitch with the Kenwood TM-D710GA transceiver installed at a local hospital, but be able to switch to using the handheld microphone and an external speaker at a moment's notice. This adatper fills the bill, permitting me flip a switch on the external speaker when someone standing behind me needs to hear the radio, or turn the speaker off while listening throught the headset. (I modified a Midland Model 21-406 loudspeaker by mounting an on-off switch on the side of the enclosure.) The headset's boom microphone and the handheld microphone are both always connected to the adapter. Normally, the handset microphone is wired straight through the adapter to the radio, so I can use the hand mic's controls and DTMF kepad. When I step on the footswitch to key the radio, audio from the headset's boom mic is switched into the circuit. My headset of choice is the Yamaha CM-500, widely available for about $60.
Several of these adapters have been placed in service. The design could be adapted to work with other radios.