When out in the trapeze on a broad reach I'm typically straddling the rear cross bar, which is just about as far as my present trap setup lets me go.
First question...why are you out on the trap on a broad reach? Are you running a spinnaker? If so, then I don't know what to tell you as I've never run a chute on a H18. If you're not running a chute, then there should be no reason to be out on the wire on a broad reach.
So what's the proper response when I does occur?
Usually the only thing you can do is shake the helm back and forth to re-attach flow. Also sheet out to unload the helm. That's what we would do on the 20 at least.
And on an aside - how important is it to raise the daggers when on a broad reach?
From a racing standpoint, raising the boards allows the boat to slip downwind and also reduces drag, so it helps in getting to the leeward mark faster. Raising the boards also helps keep the boat from flying a hull or "tripping over itself". Raise the boards in higher wind for more control downwind. In lighter wind, you can keep them down if you want to do the "wild thing". But nobody does that on an 18 since it's not faster than sailing downwind the normal way.