I don't think its a very good idea but I wouldn't say it was illegal. One of the main parts of a pelham and therefore the reasoning behind using one is that you have a curb chain which should lie flat in the horses chin groove and is secured by a lip strap (something that you do not see very often being used by pelham users). The flash noseband would intervere with the curb chain so is really pointless. If your horse needs a flash noseband to stop him from opening his mouth have you considered that his teeth may need attention. You can buy a tongue strap for him which fits onto the mouthpiece of the bit and prevents the horse from sticking his tongue out of the side of his mouth if that is the problem with your horse. Or you could try a raised port mouthpiece which is what I use for my DWB as he has a fat tongue!