Well I found one way to bring these troublesome critters out... anti aiptasia stuff puts them into fits.. got tired of the worms avoiding the trap, so I decided to have a little experiment.
I used some Stop Aptasia (all natural reef safe green goo) on a hole one of the larger fireworms was hiding in (had the stuff out to take care of an anemone that had sprung up).
The worm came out IMMEDIATLY and was none too happy, but didn't do anything but wriggle around trying to get the stuff off itself.. easily removed at that point.
While it's not the intended use for this product, it does work and I imagine wouldn't be anymore detrimental to the tank then if you were following the instructions to take care of prob anemones.
Hope that helps (and hope it doesn't qualify as terribly evil)
Edit -
This method does indeed work well.. it ticks em off to no end to have that stuff sprayed in whatever hole or cranny they are staying in.. just removed a foot long sucker using this method.. they evacuate the rock and are quite easy to catch.