I’m just looking at this SOM, with and without the heat-sink I don’t foresee this to be a clean and proper solution.
Besides, with the level of work needed to break out 5V from the SOM, I would recommend opting for re-working the PCB of the Jetson carrier board to break out power supplied to the DC jack just to avoid making messing with the SOM. Assuming any added micro-controller solution has power conditioning circuitry, this should offer little resistance, and you would not risk over-drawing from the 5V line off the SOM.
Like you said, NVidia is keeping the SOM circuitry a secret, so why try to guess what level of current draw is safe.