Still a little bit of work to do, but this is the gist of it:

A uMesh user can press the module down onto this sled.

There are headers for the uMesh module's castellated edges, some more typical 0.1" headers more towards the outside for a user to do whatever they like with it, and, interestingly, also programming headers to plug a bare ESP32 module into. Specifically the ESP32-WROOM-32D, so people can use this as a module programming jig.

There is a CP2102 USB-UART bridge and the standard programming circuitry to make up for the uMesh module having none of it. You just plug in through USB, drop your module onto this sled, and start programming. The buttons are there for if you're doing something special, but you shouldn't need them.

One issue / feature / errata with this is that it doesn't have a 3.3V regulator on board. It takes 3.3V from the uMesh module when it's pushed on. I still have to ponder for a bit, if this is okay. It does mean using it with a bare ESP32 module would require someone to supply 3.3v externally, too.

One possible solution is the CP2102 has some sort of internal LDO regulator. I'm not sure if I can use it, and it especially seems like a bad idea with the currents the ESP32 can pull. I'll hit the datasheet to see if it's viable, I guess.