I realize this is pretty niche since this pertains only to running CircuitPython/Blinka boards from Adafruit, but in case anybody else runs into this problem, here’s the solution:
First install the adafruit extended bus package:
pip3 install adafruit-extended-bus
then in your code, instead of using the usual busio way of opening your i2c connection, like this:
i2c = busio.I2C(board.SCL, board.SDA)
use this line instead, with the 6 here indicating the /dev/i2c-6 where external things connected to the Navigator board end up:
from adafruit_extended_bus import ExtendedI2C as I2C
i2c = I2C(6)
then go ahead and connect to the board or boards as usual:
ads1 = ADS.ADS1115(i2c, address=0x48) #or 0x4a or whatever you've set the board to