As I understand it, that's a very cut back version and doesn't include the compiler/interpreter so you can't just hook up to a UART and start reprogramming. I could be wrong, so please feel free to correct my statement if it's completely off the wall.

What I want is something that I can hook up to the UART serial port, and start typing interactive commands into it to debug why my system is hung/stuck/etc. It needs to live in about 4K of RAM (and I can trade quite a bit of flash to do that).

16K of RAM is too much. Something like a Nordic nRF52811 only has 24K of RAM, and a lot of that gets used for the Bluetooth stack.