You "can" hold onto the upper reciever or the assembled upper and lower, or the handguard. None of those are recommended tho. The channels that orient the barrel are not very deep. The handguard could break from twisting. These parts are VERY tough and should be fine. You could always get some vise jaws to hold the barrel.
It your tool long enough to stick out of the handguard. I'm supposing it is, cause you said you were going to grip it with pliers or whatever.
Good luck, be safe and let us know how quickly Smith sends replacement parts.