I think I'll work on OTA of the front panel MCU first.
The front MCU is a STM32L431 with 256 kB of flash and 64 kB of RAM.
Flash is organized as 128 2 kB pages, so (unlike the main processor) I have plenty of granularity for exactly where I want to put the bootloader vs the application.
The front panel does not have any persistent configuration storage, all of its settings are pushed each boot from the main processor.
The front panel firmware is small enough (38K stripped ELF) that it should easily fit almost anywhere. Hmm...