You could do all sorts of damage by riding in ANY discipline with a loose shoe, but hunting is extreme! Is there not another farrier around who can at least tighten the clenches? If it's only loose at the heel, this might be all it needs.
It can be very sticky work hunting, specially in this wet weather, its bound to loosen it, if not loose it.
Gentle hacking then yes maybe but hunting...I woudnt do it, it could twist round and injure your horse, fly off and hit someone elses or just simply damage the hoof.
Its always exciting going hunting and its very easy to think yeah it will be fine, but I guess you just have to ask yourself, will there be plenty more times that you can go, with all shoes fully fixed on so you dont have to risk anything or worry about anything?