Sure ,go in with a pair of bone cutters and nip away what you want , if the rock is a round boulder type rock then make some slots using the cutters and force a flat top screw driver in there and wedge it apart , liverock is very soft .
As for not breathing in the fumes I doubt there would be any if fragging in the tank and paly toxin by its self is not airborne so it would need to be converted to that state like boiling water to steam for example.
If you can remove it would be a lot easier.
I frag hundreds of zoas at a time sometimes and they are out of water for lengthily periods of time and watered down if I feel like its been too long.
If you can't find bone cutters a pair of wire cutters with a sharp nose does the trick just fine.