I honestly don't know, but I don't see why you couldn't rig up a little switch box.
It's not something I have to mess with on the FT-8800, which has a data port on the back so I never had to use the mic & speaker to connect. My guess is you'll end up unplugging the TT when you want to use the radio for voice.
The keyboard sounds pretty cool, have to let me know how the adapter works. The keyboard I got was something like $1.98 at Microcenter because who still uses PS2 ones? Apparently hams are the only ones. :-)
'91 Pickup - Imelda
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"To live is to suffer, to survive is to find some meaning in the suffering." -- Friedrich Nietzsche