...as Darrell explained, there really is not currently a simple way to do this
Why? if you look at the source of FormLayout you will see that the field label is created as part of the template and constructed with the label embeded - look at line 123 the label is item {1} whereby it is inserted into the string at runtime when the form is created.
Darrell would need to refactor the label field to be a distinguishable element in the DOM - its currently not...