Update firmware of one espruino device from another espruino device?

Apparently the pixl will hang for about 30 seconds after I think a socket disconnect

Yes, I'm afraid that isn't fixed yet - it's on my list of things to do :)

how can I update the firmware without having to connect to the IDE?

For the STM32-based boards, if you have a serial connection to them then you should be able to update the firmware using a tweaked version of this code which is what the IDE uses. The nRF52's bootloader on the Espruino boards doesn't support serial flashing though so you're out of luck there I'm afraid.

Another option would be to use the SWD connections and implement the SWD protocols yourself (which should work on any ARM), but that's probably quite a big project (although it would be amazingly cool).

Basically it's possible, but there isn't going to be an easy way I'm afraid.

On the other hand, updating JS on the fly should be pretty easy :)