The EC supports 2 types of I2C passthru:

1) Via host command (EC_CMD_I2C_PASSTHRU)

2) Via USB (common/usb_i2c.c)

The former supports filtering buses and address ranges.  The latter is an unrestricted passthru to ALL I2C buses.

It would be better if the USB passthru supported filtering the same way.  We should only need to open up a minimum size hole to do things like monitor INAs or upgrade the trackpad.