I don't know of a way to animate a center zoom only using the watermark filter. But it is possible to animate a center zoom using the affine transition. You would need to use two tracks: one with the background video and another with the text to be animated. Here is an example:
melt color:blue in=0 out=320 -track welcome.jpg in=0 out=320 -transition affine valign=middle halign=center scale=1 fill=1 geometry="0=10%,10%:80%x80%:0%;60=0,0:100%x100%:100%;260=-10%,-10%:120%x120%:100%;320=-10%,-10%:120%x120%:0%"
The key is that the position of the watermark is relative to the left corner. So, in addition to animating the size, you also need to animate the x and y position as shown in the example.
Also note that the animation will interpolate the size and position for each frame - but the position is rounded to the nearest pixel. So the motion may not be smooth. That is a known limitation to the animation capabilities in MLT.