I guess you probably already figured out how to access, but still posting this for completion. Yes you can use the I2S port as gpios with the TX2.
Pin of interest for GPIO mapping is GPIO3_PBB.00 / AUDIO_MCLK for 12S.
To get the GPIO number you have to look in the Jetson-TX2-Generic-Customer-Pinmux-Template.xmsl after the Pin name. In this case, it is the AUDIO_MCLK pin with the name A9 this means the GPIO is J.04
The next step is to look in the tegra186-gpio.h which is located on the Jetson at /usr/src/linux-headers-4.4.38-tegra/include/dt-bindings/gpio : #define TEGRA_MAIN_GPIO_PORT_J 9
Then you have to calculate the gpio number with the following formula:
If the Pin is a Main GPIO: 320 + (X8 + Y) = 320 + (98 + 4) = 396
So, the pin that you need is 396.