Using an inertial measurement unit to compute global motion optical flow of video
If by optical flow you mean the movement over time that the board does, it's possible to do that; I implemented a tracking algorithm using a Kalman filter over a STM32 microcontroller to get the orientation of an IMU, but it's a big amount of computation and I'm not sure that it could reduce the load compared to image processing.
Edit:
I actually think it could be feasible, and in fact it's what camera manufacturer do with image stabilization.
Caveats:

It's not that hard to do it, but I believe it's quite hard to do it well, or at least in a meaningful way;

IMUs are typically oriented to slow-ish and accurate tracking, instead you need to compensate higher frequencies of motion, like vibration and shake. You'll need to tune your tracking accordingly.


In short, you have to convert the x/y/z accelerometer axes and pitch/yaw gyroscope axes to the same set of coordinates, and apply the desired amount of filtering. Then integrate twice the accelerations to get the (linear angular) speed and the velocity of the movement, as desired. That's what you then have to compensate in the video.
You'll probably have to compensate some delays, either in the video or in the sensor readings, to time-match the compensation with the recording.