If I've declared a military challenge with Wex Pyke, can my opponent trigger Horseback Archers's ability to put him into play as a defender? I'm leaning towards yes, on the ground that there's no check to see whether the Archers have a crest before their ability activates and at that point they're then already a defender, but it'd be nice to have it confirmed for the future. Cheers!

You can ues the horseback archers, although not for the reason you list.

The Horseback archers abilty puts them into play, knelt as a defender. It does not "declare them" as a defender. This is the same reason why Desolate passage doesn't interfere with Horseback Archers (or The Wall, or Jumping Catelyn) - all of them are skipping the "declaration" step (and therefore also, incidentally, don't require icons to participate - although in the case of Catelyn/Archers, their ability limits which challenges they can jump in to).