I'm surprised we haven't seen a cheap, USB powered Bluetooth RE-232C dongle for home assistant yet.

Esphome+hdmi breakout board+llm (or use an existing project like [0])

[0] https://github.com/Palakis/esphome-native-hdmi-cec

Say if we have a pi laying around, could we emulate this?

I guess we could.

Using a whole Raspberry Pi seems like overkill, though: Support for a wide variety of common USB serial chips is already built into esphome. https://esphome.io/components/usb_uart/

Just wire it up with a cheap dev board, build a firmware image for it, and that end of things is... done, right?