Is this exposed to userspace yet? If so, I'm happy to make powerd do it when necessary -- just give me the commands to run or sysfs files to write to or whatever.

I think powerd already does this:
https://chromium.googlesource.com/chromiumos/platform2/+/master/power_manager/powerd/policy/keyboard_backlight_controller.cc#445

#2: The existing powerd code writes "0" to the brightness file in sysfs. Does that get translated to EC_CMD_PWM_SET_KEYBOARD_BACKLIGHT or EC_CMD_PWM_SET_DUTY further down the stack?

The kernel driver the ACPI interface for keyboard backlight, not the EC commands, but it should be the same in the end.

Okay. I'm assuming that this already works then, but let me know if you see cases where it doesn't.

Sorry for the false bug, this should already be working.

If we want to bikeshed, I would argue we need to disable keyboard backlight in tent mode as well. 
The short term solution would be to have chrome issue ApplyBrightnessPercent(0), 
a longer term would be to wait for the inclinometer sensor and have powerd listening to lid angle changes. 

ApplyBrightnessPercent() is a function in powerd, not in Chrome. Chrome doesn't do anything backlight-related.