How do I give an overlay a priority lower to that of hl-line-mode?

[Clément Pit--Claudel]

Hi emacs-dev,

A mode that I contribute to uses overlays to highlight certain lines. The added 
overlays typically cover (point-at-bol) .. (1+ (point-at-eol)), which 
unfortunately does not play nicely with hl-line-mode: my overlays hide the one 
added by hl-line-mode.

Demo:

(with-current-buffer (get-buffer-create "*hl*")
  (erase-buffer)
  (fundamental-mode)
  (hl-line-mode)
  (dotimes (_ 5) (insert "AAAAAAAAAA\n"))
  (goto-char 25)
  (let ((ov (make-overlay (point-at-bol) (1+ (point-at-eol)))))
    (overlay-put ov 'face '(:background "red")))
  (pop-to-buffer (current-buffer)))

Of course, the temptation is great to add a little (overlay-put ov 'priority 
-100), but it's explicitly a bad idea:

> This property’s value determines the priority of the overlay. If you
> want to specify a priority value, use either nil (or zero), or a
> positive integer. Any other value has undefined behavior.

What alternatives do I have? (In fact, do I have any?)

Thanks!

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