This appears to be somewhat of a u-boot problem, and not specifically tow-boot, but I figure an opinionated configuration might provision for this sort of thing:

I had Tow-Boot installed to SPI, and I was poking around the console, and I tried running saveenv, which proceeded to save environment settings to SPI, overwriting part of the loader. Which part? I don't know. It didn't say where in SPI it was saving, but I presume it was an important region because now the power LED is red upon power on and nothing else happens.

Maybe saveenv can write to a specific offset past the bootloader? I'm not sure what the best course of action is here. All I know is that I rendered my Pinebook Pro unbootable through a command with a relatively innocuous-sounding name. (Yes, I should have looked up what this did first…)