A tiny dab of liquid Frontline on a cotton bud should do the trick, it will then die and eventually drop off, and the Frontline will only be on the tick and not the hen herself, so probably no egg withdrawal problems. Or you could use a tick hook, twisting gently to unplug the tick rather than pulling it off. You'd probably need a second person to hold her for you if possible. Of course she may have other ticks elsewhere, hidden under her feathers, quite a problem finding them.
I've never noticed any ticks on my hens, probably because they're kept in a covered run with no access to long grass and hedges etc, but on our dogs, before the invention of spot-on treatments such as Advantix that prevent ticks as well as fleas, I used the methods above. Or you could try simply coating the tick with Vaseline - a bit messy on the feathers and would attract dust etc but would help to smother the tick so it couldn't breathe and should die.
