Hi,
this is not that complicated. Just add a script in the exit event of the dropdown to update the fields caption:
var oField = xfa.resolveNode("form1.#subform.SignatureField1"), // reference the signature field
cValue = this.rawValue; // the current selection of the drop down
oField.caption.value.oneOfChild.value = "You've selected " + cValue; // modify the signatue fields caption