Bangle.setLCDMode("120x120") in emulator

Does the display controller support and expose copy commands and have memory that can be written to but is not displayed

Yes - it's got a 240x320 buffer and that's what I use for 'doublebuffered' mode (although it requires going widescreen to 240x160), scrolling, and also when rendering notifications.

can the display controller be configured to use only a portion of its memory and scale it up to the full size of the display

Not as far as I know. The display is the ST7789 so if you can find any interesting hacks I'd love to hear them: https://www.rhydolabz.com/documents/33/ST7789.pdf

Bangle.js provides a Bangle.lcdwr function for writing commands to the display, so it's actually possible to try out stuff yourself from Espruino when you have a Bangle.js :)