Be very careful with any manual removal of zoas and paly's due to the potential of palytoxin. I honestly would only use this method if I could pull them out fo the tank first and do the manual removal in a separate bucket with glasses, gloves on and mouth shut(or covered).
It may end up being the only way to remove them though, and I would just be extra careful, for the most part it should be alright but with something as dangerous as palytoxin you cant be too careful.
That smothering idea may alwso work, if you could cover them with an epoxy, maybe even one that can be removed after they are dead. Too bad they are in a place you cant remove from the tank.