Here is a sample for a text field with the fully-qualified name form1.page1.subform1.tf.
xfa.resolveNode("form1.page1.subform1.tf.caption.value.#text").value = "Favourite Soccer Team";
Sample attached. Note, it is possible to do bind captions to source data.
ChangeCaption.pdf 78.3 K
The only option would be a macro, which requieres Designer ES2 or ADEP Designer.
Here's a macro with a flex dialog I designed to manipulate fields at design time: