I asked AMD's David Glen, one of the engineers behind FreeSync, about how AMD's variable-refresh scheme handles this same sort of low-FPS scenario. The basic behavior is similar to G-Sync's. If the wait for a new frame exceeds the display's tolerance, Glen said, "we show the frame again, and show it at the max rate the monitor supports." Once the screen has been painted, which presumably takes less than 6.94 ms on a 144Hz display, the monitor should be ready to accept a new frame at any time.
What FreeSync apparently lacks is G-Sync's added timing logic to avoid collisions. However, FreeSync is capable of operating with vsync disabled outside of the display's refresh range. In the event of a collision with a required refresh, Glen pointed out, FreeSync can optionally swap to a new frame in the middle of that refresh. So FreeSync is not without its own unique means of dealing with collisions. Then again, the penalty for a collision with vsync enabled should be pretty minor. (My sense is that FreeSync should just paint the screen again with the new frame as soon as the current refresh ends.)