"Execution Interrupted" in interpreter

is the flash storage on the Bangle memory-mapped in one flat chunk?

Sadly no... The interpreter makes the flash appear memory-mapped in most cases (eg accessing a file in Storage doesn't load it into RAM), but that's all done in software - so Inline C won't be able to access it directly.

If you're really into doing low-level stuff then right now there's a few 100k of unused internal flash in Bangle.js 2 before address 0xf6000. So you could use the Flash module to access that directly.

On Bangle.js 2 there is actually the potential for memory-mapping the external flash too using hardware, but that isn't done at the moment.