This takes each subpath and reverses it. An example is shown below. The first animation is:

<ellipse rx="5" ry="3" style="fill:blue">
<animateMotion dur="30s" begin="0s" rotate="auto" repeatCount="indefinite">
<mpath xlink:href="#ausoutline"/>
</animateMotion>
</ellipse>
<path id="ausoutline" d="M390.94,316.70 C391.09,316.40 391.04,314.25 391.14,314.00 
...1300 more cubics...
389.54,317.00 390.49,317.15 390.94,316.70 Z"/>

The second animation has the reversed path definition:

<path id="ausoutline" d="d="M 390.94 316.7  
C 390.49 317.15 389.54 317 389.34 316.8 
...1300 more cubics...
C 391.04 314.25 391.09 316.4 390.94 316.7 Z "/>
Original Curve Reversed Curve

9.1 Reversing a path