After several weeks of heated debate among fellow judges — all far more experienced than I, but no less stubborn for it — we have agreed to refer the matter to a higher authority.

In the attached diagram, assume that blue is L and yellow is W. Both boats (one-design) are on a downwind leg, approaching the leeward mark, and their proper course is about 135° TWA.
L became overlapped to leeward of W from clear astern, so RRS 17 applies. 

The question is this:

If L bears away to 180 degrees to the true wind — dead downwind — without gybing, is L sailing above her proper course for the purposes of RRS 17?

My understanding is that RRS 17 only prevents L from sailing above her own proper course, not below it. And I believe it is generally accepted that sailing at 180 degrees to the true wind is sailing below, not above, than sailing at 135 degrees to the true wind. So, in principle, L should be allowed to bear away and sail dead downwind without gybing.

In other words, Rule 17 is not a general rule against L making life difficult for W. It is narrower than that. When it applies, it prevents L from sailing above her own proper course. It does not require L to sail exactly on that course, and it does not prevent her from sailing below it.

We would be curious to hear whether the forum agrees with this reading, or whether there is a rule I am overlooking that would lead to a different conclusion.