You can do this with a few lines of code.
Assuming you have a subform with a positioned layout that contains a multi-line textfield which should grow from bottom to top. All you need is the following script in the calculate event of the subform.
var nSubformH = parseFloat(xfa.layout.h(this, "mm"), 10),
nFieldH = parseFloat(xfa.layout.h(this.Textfield1, "mm"), 10),
nDiffH = nSubformH - nFieldH + "mm";
this.Textfield1.y = nDiffH;
xfa.layout.relayout();