You can insert any field manually if you know what the syntax of the field
is. Press Ctrl+F9 to insert the field delimiters and then type the text
between them. Updating of fields is always a problem in protected forms.
Usually a macro of some sort is required (but don't ask me about about
that!).

I'm preparing an article on this and other uses of StyleRef fields, but it
won't be published for a few days, so here's a summary. You can use a
StyleRef field to pick up text in any given style and repeat it elsewhere on
the page (the header or footer is the usual location). The style referenced
can be either a paragraph or a character style. A character style doesn't
have to differ from the default paragraph font in anything but name; you can
apply a character style that does not change the appearance of the text at
all, just marks text to be picked up by the StyleRef field.

So if the data you want in the header appears anywhere on the page (for
example, if it will be entered in a form field), you can apply a specific
style to that form field and put a StyleRef field in the header to pick up
that style. The text will appear in the header on every subsequent page
until the field detects other text formatted with the referenced style.
Obviously, if you want the text to appear on *every* page, you'll need to
have the form field that provides it on the first page of the form.