BMA421 Accelerometer

Even powering down the watch doesn't seem to bring it back

Did you actually power it down by removing the battery? In these devices everything is always powered even when the watch is off, so if you'd configured the accelerometer you wouldn't expect it to reset until the battery was removed (or totally flattened).

any reads from the BMA421 registers just return 0xFF

Could be the I2C address has been reconfigured? Or it's been put into SPI mode?

You could try accessing it by any other I2C addresses it might have