After a bit of probing and experimentation I found suitable points to connect TTL level TXD/RXD into the points that formerly handled the 20mA current loop connections. I pulled out the opto isolators and just left the current loop circuitry hanging. The RXD signal seems to be an open collector and sources barely a few uA at high logic level, and whatever the TXD is, it accepts a 3.3V high alright, so I have just hooked them straight onto a 3.3V Raspberry Pi for testing.
The long range project is to embed the Raspberry Pi with it's software stack right into the terminal. Although there is a lot of room inside the case, most of it is open to the high voltage fields of the CRT. The logic circuit is squeezed in next to the outer case fairly closely, but I believe I have room to stick a Pi-zero in there with some double-sided tape on top of the EIA-RS232 transceivers. The SDCard storage on the Pi is pretty slow, but even so I have SIMH running a Microvax and VMS 7.1 and the overall performance feels like a VAX750 I used to work on. The Pi has more memory, storage and a faster CPU, but the storage speed is really awful, even for an old VAX.
I wanted to customize the strings in the VT220 setup screen to make the setup screen reflect the repurposed 20mA current loop network connection. I was able to read the ROM in my TL866 and have been poring over the 8051 code. I can find the strings alright, but they are broken up and rearranged strangely like the string package has been compressed somehow. I do not believe I will be able to accomplish my mission here without discovering the exact encoding algorithm that's been used and that is likely lost in history at this point.
My remaining tasks are to tinker with U-Boot a bit more and the FreeBSD init process to get the startup of the Pi software to work right. Once this is done I should have a somewhat amusing nostalgia piece packed into the stand-alone VT220.