if you want to do it without changing the level, it's going to be hard. Try to change some actor/thing flags, i think there are some for invulnerability, but i'm not sure if they will work, and rechange them after a timer has timed out. You could change the flags and start the timer in the weapons cog that does the kickback.
If this does not work, there is another way i can think of. Try to create a thing with a bounding box a little bigger than the one of a player. Then have in the weapon cog this thing created and attached to the player. Maybe it will work as a shield, who knows (of course, you would not want to create a 3do for it, and you would want to destroy it shortly after it's creation). Tell me if you managed to do one of those things...
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