During a seizure it's virtually impossibe to do anything. It's very hard to put anything between their teeth and if they get a lock-jaw, it's just not possible.
You could also injure yourself or break the victims teeth.

The best thing to do is to make sure the victim doesn't hurt himself to objects aroud them, and wait for the seizure to stop.
Then you can put the victim into the recovery-position to prevent blood from going into the airway (if the victim remains uncounsious ) and try to stop the bleeding.