I can see this is an old message but thought I would respond.
I have a FT-8900R and plan to look into tapping the discriminator output.
I have tride using the 9600 baud data port on the back with limited luck.
Looking at the schematic I can see they used 2 IC's for the discriminator.
Side LEFT and SIDE right of the radio. Both sides can receive different freqs at the same
time. The choice would be determined by what side will be the freq you want to listen
to or use. I just got this radio and from what limited reading of the manual and playing
I noticed the 800MHz band is only available on the LEFT side. Saying that I plan to explore
IFA Q1024 which is one of the 2 TA31136FN discriminator ICs. I looked this chip
up and pin 9 appears to be the unfiltered ouput. The output connects to a 1K resistor.
the other side of the resistor goes to a .001uF cap and then to Test Point TP1020.
The .001uF goes to ground and I think it might effect the audio so going straight to pin 9
of the chip might be better. The test point might be easier access but I don't know
the effect of the .001uF cap going to ground which is prior to the test point.
My use will be for DSDplus decoding APCO 25 on 800MHz.