In the recently released public beta version of the FP6, that's possible by a slight modification of the "newly added" "Z Scale by Include Boundary" Effect (to operate on the Y axis) in the following way:
real proximityToBoundary = if(fpItem.areaIncDistance<FalloffDistance, (fpItem.areaIncDistance/fallOffDistance),1);
fpItem.scale.y = (fpItem.scale.y*evaluateCurve(ZScaleCurve,1-proximityToBoundary));
Attaching a sample scene. Hope, you'll find it useful.