You could format the last paragraph in the header with a bottom border; that will run all the way to whatever indents that paragraph uses. If you want a full-width line but your existing last paragraph has left/right indents that don't align with the margins, you could add a new last paragraph whose left/right indents do align with the margins. That new last paragraph could be formatted with as little leading/trailing space as you require and be given as little as a 1pt font size to minimise whatever other visual impact it might have.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]