Since Klipper doesn't send G-code to the physical printer's serial port, it needs to receive it on a host machine for processing before forwarding on in it's preferred format. To do this, it creates a virtual serial port, typically named /tmp/printer. OctoPrint then connects to this serial port instead of the more-typical /dev/ttyUSBx, etc.

My plan is to use a pair of USB Serial-TTL adapters connected together, to provide a physical point I can connect the TFT28. OctoPrint will connect to one of them, and I'll use something like socat to connect the other to /tmp/printer. It should look something like this:

I'm not 100% sure the AUX port on the GEN_L doesn't mirror both TX and RX from the main serial port (USB) to the TFT28's RX, if that's the case I'll have to come up with something different. Probably something smarter. But this is the current plan as it's easy to try with what I've got lying around.

Note: All my experience with Klipper is from the perspective of an OctoPrint user, I understand it can be used without it, but as I've not run it that way, I've probably mistakenly attributed some things to one that belong to the other.