I made a little progress over the weekend.

First, I rigged up my ID-5100a with an RJ45 splitter between the mic and the radio base so I could tap into the 8Vdc, PTT, and mic audio pins to observe these signals during normal operation.  The 8Vdc and PTT signals made sense, but I'm a bit baffled by the mic "audio" signal.  I expected to see an obvious "line level" audio signal, proportional in frequency and amplitude to the volume of my voice, but I saw nothing of the sort.  Instead, the voltage between the Mic Input and Mic Ground pins was basically 8Vdc plus a 300 mV pk-pk signal in the 2 MHz range.  What the heck?  Zooming in it does not look like some sort of digitized serial stream.  I scoped the other wires but couldn't find anything resembling an AF signal while I was talking into the keyed mic (and also hearing my own transmission, received on an HT).  What gives?  Still kinda baffled by this. 

Anyways, I see that Signalink jumpers the ID-5100a Mic Ground to common ground with PTT ground, and wires the Signalink MIC (output) pin directly to the ID-5100a Mic Input line.  I can't find an explicit description, I presume the Signalink just sends line level AF through that connection.  I might ask the ID-5100a facebook group about this since several guys in that group use Signalink with this radio.

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I found several Arduino KISS/AX.25/APRS examples online, but none were implemented in a way that was intuitive to me, so I started down the path of defining my own KISS library and AFSK mod/demod elements.  I'll borrow bits and pieces from other projects to get forward error correction and other functions working quickly.

Starting from scratch, it turns out SerialBT is super easy to work with, and persistent, so I can reflash and test incrementally without my PC dropping the BLE association.  I easily set up Winlink in Packet mode through KISS interface, and pointed that to the serial com port of the ESP32 BLE receive channel, received KISS messages on the ESP32 and echoed them back to the USB serial interface.  The KISS TNC control messages immediately made sense but I haven't yet worked out the first data message which I presume is some sort of AX.25 CQ message.

Screenshots later..