hackish: real-time random watch face

It seems this is essentially having the same effect?

It still reboots, however it's a lot tidier than having the code in the bootloader (IMO).

it seems rather trivial to screw this up in any given clock app (even if there is an unload function), leading to leaks...

Yes.. That's what led me to the current situation of full reboots for each app - it's just way more stable when you don't completely trust an app.

However in most cases with clocks I think it'd be pretty possible, and it would be quite neat and would enable features like swiping to new clock faces.

modifying my own firmware?

This is the best link: https://github.com/espruino/Espruino/blo­b/master/README_Building.md

It's actually pretty straightforward - especially on Linux or Windows (with WSL installed). The bootloader is designed with a watchdog to try and recover if a bad binary gets flashed to it, so it's not too painful to develop (although I develop with SWD wires attached as it makes it much quicker to flash new firmwares)